WHEN BOB Keenan was young he joined the Royal Marines and served his country. In 1991 he was living in Bristol, his back damaged during the decade he had spent as a fireman after leaving the army. "I was an inquisitive bugger," he tells me over lunch, "always trying to find out how I could be better at what I did. In some instances, I would be scared or apprehensive, or have some physical reaction that would stop me from doing something. In the fire brigade a split-second decision can be the difference between someone living and dying. And I wanted to be able to handle that. It was primarily philosophy I was looking at. Then one day someone put a leaflet through my door."

The leaflet was promoting Dianetics by pulp novelist-turned-self-help-guru, L Ron Hubbard. Published in 1950, the book outlines Hubbard's thesis for self-improvement and self-awareness and introduces some of the central planks of what would become Scientology: that hidden in our subconscious or "reactive" mind are traumatic memories ("engrams"); that these must be removed by a process called "auditing" if we are to achieve the state known as "Clear"; that only when we become "Clear" can we fully realise our potential as human beings.

"Ironically, I'd never seen all the crap that was written about Scientology," says Keenan. "I read the book and thought That makes sense'". His conversion to Scientology had begun.

That "crap", as Keenan calls it, is a handy catch-all for every biography, exposé, newspaper article, website and South Park episode that has ever attacked the Church of Scientology, its founder, its practices and its adherents - people such as Tom Cruise, John Travolta and, at least according to some recent US press reports, David and Victoria Beckham. These attacks started on August 15, 1950, with a review of Dianetics in American magazine New Republic which called it a "bold and immodest mixture of complete nonsense and perfectly reasonable common sense, taken from long acknowledged findings and disguised and distorted by a crazy, newly invented terminology". And it continues today, with websites such as Operation Clambake, which calls Scientology "a vicious and dangerous cult that masquerades as a religion".

Of course one person's "crap" is another person's truth and, while the church dismisses its critics as acting out of vested interest or sectophobia, or writes them off as disaffected former members or out-and-out troublemakers, their voices need to be listened to.

In an effort to sift fact from fiction, I've come to the Church of Scientology's massive new London HQ to talk to three of its leading British members - about their beliefs, primarily, but also about their reaction to the various controversies that swirl around this curious institution. Do they really believe in aliens? Why do they venerate Hubbard to the extent that he is mythologised and his achievements (to my eyes at least) embellished? Do they accept that free speech allows me the right to mock or is mild scepticism all I am allowed? And what's behind the hatred of psychiatry?

Keenan is one of my three guides. He is "Clear" and informs me that he has an IQ of 148 (Scientology claims to be able to raise a person's IQ; 148 is very high, just a little way behind Carol Vorderman). Now 47, his back pain is cured (a pleasant side effect of Dianetics, he says) and he has left the fire service behind him. Today he is director of the L Ron Hubbard Foundation which oversees Hubbard's vast literary legacy.

The others sitting round the table with me are 51-year-old Janet Laveau and 43-year-old North Berwick-born Graeme Wilson. Both have been Scientologists since their teens, Laveau entering the church while studying medicine in her native Canada, Wilson while a fresher at the University of Edinburgh in the early 1980s. He found Scientology via its Scottish centre which has operated from the same building on Edinburgh's South Bridge since 1968.

Wilson handles public affairs for the church and Laveau is head of external affairs for the UK. All three are based at the Saint Hill complex in East Grinstead in West Sussex, formerly Hubbard's home and now Scientology's UK administrative headquarters. The church claims 123,000 members in Britain and some 10 million worldwide, up two million from 1997. However, these figures are disputed. The most conservative third party estimates put the worldwide membership at 500,000, and some have it as low as 100,000.

Later I will also meet 27-year-old Mark Pinchin, a former drug addict who helps run the London centre. Dressed - like most of the 75 staff here - in a dark suit and red tie, he will show me the auditing rooms which contain the E-meters: face-and-dial contraptions that measure the body's electrical current. He will also show me the office that is set aside for L Ron Hubbard.

Every church has one of these and for some it's proof of yet another wacky Scientology belief - that Hubbard will return one day, that when he does he'll need a desk and a blotter and a comfy leather swivel chair, and that until then the room must stay hermetically sealed. In fact, it's open for all to see and, says Pinchin, intended simply as a celebration of Hubbard's achievements as a writer. And are people allowed in? "Of course," he says. "I mean someone has to clean it."

Finally, I'll see the shop where CD and book sets sell for up to £90. There are a lot of them - L Ron Hubbard completists would find Scientology an expensive habit to acquire.

First, though, let me rewind a couple of hours, to the moment I arrived outside the front door of number 146, Queen Victoria Street, formerly the home of BP and once the headquarters of the British and Foreign Bible Society but now adorned with the flag of St George and the logo of the Church of Scientology.

It's here that I need to take stock of the prejudice and bias I will check in at the door. I've promised Keenan, Laveau and Wilson a fair hearing. I want to find out what they believe. To do that I have to jettison the opinions about Scientology I've gleaned from watching South Park, from reading those biographies and newspaper articles, or from looking at the many websites that attack the church. I have to prepare myself for the possibility that everything I see might actually be true.

Getting inside Scientology's new multi-million pound centre was easier than I thought. I just rang up. No problem, they said. Come for lunch but be prepared to spend the day here. Fine, I countered, but for your part, be prepared to answer the complaints that have been levelled against the church over the years: of criminality, of avarice, of hypocrisy, of unwarranted hostility towards those who leave the church or attack it.

No problem, they said. Come at 11am.

So here I am outside. It's five minutes before the hour and my mind is open, my scepticism is in neutral and I'm as prepared as you can be for a Damascene conversion.

My tour begins with a series of audio-visual presentations promoting the church's secular activities, primarily Narconon and Criminon (drug and criminal rehabilitation programmes) and Applied Scholastics (an educational skills set based on lectures delivered by Hubbard at Saint Hill in 1964). Anyone can come in and watch these presentations, though nobody else does when I'm there. In fact, I see very few people during my time in the building.

I sit in an armchair beside Wilson who uses an on-screen display to bring up what is essentially a series of infomercials on the huge wall-mounted flatscreen TVs. From these I learn that Applied Scholastics aims to create a "new and literate civilisation on Earth" and that under current educational procedures "millions of children are forced into psychiatric tests". One American boy says he was cured of the dyslexia a psychiatrist told him he had, through his use of "study tech", an Applied Scholastics technique. Gambia's secretary of state for education is also seen praising Applied Scholastics.

Finally, Tom Cruise pops up, filmed giving a speech to celebrate the opening of Applied Scholastics International's headquarters in St Louis, Missouri. He talks fluently, charmingly and persuasively about the difference "study tech" made to him. He is long-haired and bearded. "Must have been when he was doing The Last Samurai," Wilson murmurs.

"Study tech" is as an educational system which addresses the problems of how to learn, rather them how to teach or how to shape a syllabus. There is a school in the UK - Greenfields in East Sussex. Founded in 1981 and with with annual fees of up to £13,956, the school's prospectus states that it is non-denominational and welcomes children from all faiths. But, it adds, "we do teach a non-religious moral code as a foundation for successful living - The Way To Happiness developed by L Ron Hubbard, noted educator, humanitarian and philosopher".

The Way To Happiness is made up of 21 precepts, things such as Love And Help Children, Be Worthy Of Trust and Be Competent. I watch an illustration of Be Competent which involves a young girl who wants to be a fencer. It ends with her winning some sort of Olympian accolade. Were it not for the cheesy muzak it could be a Nike ad.

The official position on these secular activities - Narconon, Criminon and Applied Scholastics - is that they simply use the teachings of L Ron Hubbard and are otherwise independent of the Church of Scientology. However, that didn't prevent Edinburgh University Students' Union (EUSA) stopping Narconon Scotland putting up posters advertising its services. The rationale behind the move was that the union couldn't be sure Narconon was, in the words of EUSA's then vice-president Mark Calder, "independent".

"We carried out research which showed it has strong links with Scientology," Calder said at the time. "We were also concerned that this link was not made explicit so a decision was made to take the posters down."

The audio-visual presentation lasts for well over an hour. I have to say it drags a little, though I do enjoy the film about Venezualean Scientologists Audrey Cabrera and Ruddy Rodriguez. It seems it was only the personal intervention of these indomitable women that prevented the country falling into anarchy during and after the 2002 coup attempt against president Hugo Chavez. There's even a picture of the women meeting him, alongside other images of them handing out copies of The Way To Happiness.

And so we come full circle: I'm eating lunch at a round table in an ante room with the blinds pulled down; Keenan is telling me about the Royal Marines, serving his country, about his bad back and his being "an inquisitive bugger".

Time, though, for an inquisition of my own. I mention South Park, Matt Stone and Trey Parker's ribald animated series about Stan, Kyle, Kenny and Cartman, four schoolboys in the eponymous Colorado town. In episode 912 of series nine, screened in America on the Comedy Central channel in November 2005, the Church of Scientology gets the satirical equivalent of a happy-slapping. Former soul singer Isaac Hayes, who played Chef in the series and is a Scientologist, quit as a result and because of the controversy Comedy Central was forced to cancel a planned repeat in March 2006.

Keenan has seen the offending episode, but for those such as Laveau who haven't, it involves Stan taking a personality test and being told he is depressed. He raises the $240 he is told he needs for auditing and is then discovered to be the reincarnation of L Ron Hubbard. Tom Cruise turns up but hides in a closet when Stan says Leonardo DiCaprio and Gene Hackman are better actors.

The president of the Church of Scientology also arrives and reveals to Stan the innermost secrets of Scientology. These involve an intergalactic federation of planets ruled over by the evil Lord Xenu, frozen alien bodies and giant soul catchers in the sky. An onscreen caption reads: "This is what Scientologists actually believe."

The inspiration for this is supposed to derive from an official church document which is only shown to those who reach the level of Operating Thetan III, one of the levels above Clear in the church hierarchy. But is it what Scientologists actually believe? "You mean the science fiction stuff?" laughs Keenan. "It's not part of any theology you will study in the church. "This science fiction material that they continually take out of books is a classic example of the marginalising of the group. It's a sectophobic viewpoint trying to create the idea that we're a bit weird."

It works. But to be sure I ask the question again: so you definitely don't believe in the intergalactic war, in Xenu, in the volcanoes and soul catchers?

"That is not part of the theology of Scientology," says Keenan. Laveau, who has achieved Operating Thetan level III, chips in here. "South Park is a cartoon," she says dismissively. "On this whole subject of aliens, I did a test: Mr Hubbard has written 3000 lectures on Scientology and I've looked in all the indexes for the word alien. I found it once and that was in reference to a person from a different country."

Official Scientology biographies are at variance with the story of L Ron Hubbard's life as told in books such as Bare-faced Messiah by former Sunday Times journalist Russell Miller. "I've read all the Russell Miller crap," says Keenan. "I think the guy was misled, personally. I read it and I thought Where's your evidence? What are you trying to say?'."

He is equally dismissive of every other biography that questions the Church-sanctioned view of Hubbard's life. "The evidence they tend to have is often ambiguous and often has a viewpoint attached to it. I understood that and I set out to find out what people who knew him say."

Keenan has spent four years working on a documentary about Hubbard in conjunction with author Dan Sherman. He has travelled the world, he says, tracking down people who knew Hubbard, "people who aren't scientologists, everyone from nuclear scientists in America to Indians in Guam. I've found people we thought were dead. I've had the weirdest experiences."

Such as? He tells me about a former CIA recruiter he met in Washington DC who claimed he saw Hubbard coming out of a restricted area of the Pentagon sometime in the 1950s. Hubbard, says Keenan, was working for US naval intelligence. "When we talk about Hubbard's history the first thing people say is, where's the evidence?' Well we've got the evidence. I've got rooms bigger than this, full of evidence of what Hubbard was doing, including his wartime history, what he was doing in Australia with naval intelligence, what he was doing up in Alaska when he was chasing a spy up there who J Edgar Hoover was looking for."

As for the discrepancies in Hubbard's war record - most biographers claim he never saw active service with the US navy, was twice relieved of command and once shot up Mexico by mistake - Keenan has his own explanation. "I have in my possession five different records issued by five different departments in the military that say five different things about his war record. Not written by him or by us but by different departments."

And the reason? "Sheep dipping." Excuse me? "Sheep dipping. It's a standard intelligence procedure - you create several records for someone so that you can't get the clear picture. Hubbard used to say, I always told the lesser tale because I don't think a lot of people would believe me.'"

When I start talking about psychiatry, however, the mood changes. Answers become shrill and the subject seems to elicit a Pavlovian response - ironic given Scientology's dismissal of stimulus-response behaviour. It's true that psychiatry has a serious case to answer and their long campaign against Ritalin is now being echoed in the mainstream media where concerns about its use are mounting. They have also campaigned against Seroxat and Prozac. Yet it's the manner of that campaigning and the single-minded pursuit of this one subject that I find confusing.

Finally, I mention the "make a million, start a religion" maxim which is often attributed by Hubbard, and taken by knockers as proof that Hubbard developed the theology of Scientology simply to make money.

"George Orwell," my companions chorus. Wilson leaves the room and returns with a photocopied excerpt from volume one of the Collected Essays, Journalism And Letters Of George Orwell (there will be several more of these handouts over the course of our lunch as the refutations continue). It is a letter from Orwell (signed Eric Blair) to Jack Common, dated February 16 1938. "I've always thought there might be a lot of cash in starting a new religion," he writes. "We'll talk it over sometime."

"I'm sick of it," says Keenan. "The only evidence that ever came out was two guys at a science fiction convention who said they heard Hubbard saying it. What they could have heard was Hubbard quoting it."

Actually, that isn't the only evidence. A lengthy and well-researched article by Rolling Stone's Janet Reitman in 2006 dug up the autobiography of science fiction writer Lloyd Eshbach. In it, Eshbach claims Hubbard said the same thing to him in the late 1940s. Hubbard's estranged son, Ron DeWolf, said in a 1983 interview with Penthouse magazine that he too heard those sentiments from his father. Ultimately, however, it is unprovable. As claim and counter-claim flash around the world - and, increasingly, the internet - perhaps it's not that easy to separate fact from fiction.

Some facts are beyond dispute, of course: for instance, Mr Justice Latey's judicial conclusion in a case before the family division of the Royal Courts of Justice on July 23 1984 that "Scientology is both immoral and socially obnoxious. In my opinion it is corrupt, sinister and dangerous"; or the fact that in 1979 nine high-ranking Scientologists - including Hubbard's then-wife - were convicted of breaking into government offices and stealing official documents.

"Yes we've had our fair share of bad apples," says Keenan. "Of that there is no doubt. And we've expelled them when we found them out." The church has changed, he says. So why were there still people protesting outside this very building when it opened last October?

"There were four of them," says Keenan. "What about the eight million people in London who don't have a problem with Scientology?" Why were there even four then? "Maybe they've heard that somewhere, sometime, someone got upset with Scientology and they're fighting their cause. I don't know."

Laveau interjects. "Some of these guys who show up are paid. We've been told." Paid by who? "We've never found out." Why do you think you get such a bad press generally then? "We'd like to know sometimes," says Keenan. "Obviously, we're going to say to you Hubbard never said this, obviously, we're going to say to you Scientology's good. But at what point do we start to make sense? Here's a thought: what if Scientology has been maligned to stop certain areas of society - those who need drug rehabilitation, education - receiving something that would not be in the vested interests of others? What if one of the only natural drug rehabilitation programmes actually did work? Who would that upset? We don't have multi-million pound budgets to go lobbying governments."

Don't you? I thought the church was rich. You have to pay for auditing, the church seems to be run like a business and one of the major complaints against it is that it is simply a money-making enterprise aimed at enriching its top executives. Time Magazine, for one, inclined to this view in a notorious cover story published on May 6 1991 with the strapline: "How the growing Dianetics empire squeezes millions from believers worldwide." And then there's this building, a new building in Berlin Keenan sighs, as if he is weary of answering this question. "I wish I knew where this f***ing money was "

Laveau tells me that several faiths charge for particular ceremonies or even for rent of a pew, and reels off a list of Christian denominations, such as the Mormons, which expect followers to tithe a part of their earnings to their church.

And so we roll on like this for more than two hours. Mostly Keenan, Wilson and Laveau are reasonable in their answers. I find some of their more grandiose claims for Hubbard a little hard to take but I'm aware that his critics are just as selective with their use of the facts. It's their stand on psychiatry that I find most baffling. Why not get exercised about the arms trade too? Or the environment? Or globalisation? How about landmines, child soldiers, prostitution, the rainforest?

Back in Scotland, I speak to Dr Steven Sutcliffe, a lecturer in religion and society at the University of Edinburgh, who says the Scientologists' apparent fixation on Hubbard is not that unusual. "Most religions have a very revere