BERT JANSCH is not naturally exuberant. He is prone to understatement, and is sometimes happy to let his sentences drift. But he is rarely vague. "I'm a naturally reclusive person," he says quietly. "I don't like parties. I don't mind having a good time, but I'm not that sociable. I find it very hard to project myself in any way at all unless I've got a guitar."

Happily, Jansch is uncommonly expressive with the guitar, and while he made his name in the late 1960s when the folk boom crossed into rock music, and later with Pentangle, who fused jazz and folk, his extraordinary ability has made him a touchstone for subsequent generations. Johnny Marr (ex- of The Smiths), Bernard Butler (of Suede), and Hope Sandoval (of Mazzy Star) have come visiting. More recently it has been Pete Doherty and neo-folk troubadour Devendra Banhart.

Jansch joined Doherty onstage earlier this year, and played with Banhart's band, The Espers, at Neil Young's Bridge School Benefit concert last autumn. He also accompanied Young on Ambulance Blues, a song many have identified as a copy of Jansch's Needle Of Death. Young is on record as saying that Jansch was a big influence on Young's old band Buffalo Springfield, and that Ambulance Blues may have subconsciously borrowed its melody. But Jansch holds no grudge. "The opening chord is similar. That's about it."

Anyway, Jansch professes himself happy enough with the attention. When Marr or Butler come to his house in Kilburn, West London (currently undergoing renovations after a flood), they often have to teach Jansch his old songs, because he hasn't played them for so long. "It's quite good in a way for me to know what other people do like, because I can get very obscure. There have been points in my history where, with some of the songs, you'd have to have a lot of explanation as to what they were about. It was only me that understood them at all. The guitar is what I'm thinking at the time. That's what comes out."

His musical journey begins in Edinburgh. When Bert was three months old, his mother left his father in Glasgow, and they moved into a council house in West Pilton. His interest in music was sparked at Pennywell Primary School, when a teacher brought in a Spanish guitar. This was the time of Elvis and skiffle, so his attention was snared. He built a guitar from a kit, and - by now at Ainslie Park high school - was introduced to the Howff club in the High Street. The folk scene in Edinburgh and Glasgow was vibrant in the early 1960s, and Jansch believes that he and his friends, Robin Williamson and Clive Palmer (of the Incredible String Band) were hippies before the term was invented. He had tried most drugs before he was 16. "I did have sources in the university which we won't go into. But pot smoking was the mainstay of that society."

Musically, Jansch was learning folk and listening to blues: a bit of Pete Seeger here, some Brownie McGhee there. But his biggest influence was Davey Graham. Jansch was given guitar lessons by Graham's sister, Jill, who loaned him one of Davey's tapes. It included his song Anji, later recorded by Simon and Garfunkel. But Jansch had it first, on his debut album in 1965.

"Davey broke down all the barriers in music. The last gig I saw him play was in Northern Ireland, last year. He did one song called 16 Tons made famous by Tennessee Ernie Ford. I remember that from childhood, and Davey's done a fantastic version. He played Indian stuff, and for a long while he was into classical music - he's a quite extraordinary player."

Graham, says Jansch, has a photographic memory for music. "If he heard something, he'd be able to play it. Even if it was the whole LSO orchestra playing, he'd be able to interpret it and put it onto the guitar. And he did it with jazz band-leader Charles Mingus. Even with Woody Guthrie songs, the things that you would not normally associate with getting it right - a bad intonation or whatever - he would actually mimic that."

Jansch was a wild kid. In his early teens, he used to work with his older brother as a nurseryman, and on leaving school at 15, was faced with a choice: "Guitars or the garden. I'm afraid the guitar won."

From 16, he roamed, "forever hitchhiking". He and Williamson would thumb their way to London, and then to Paris, and on to St Tropez and North Africa. This lasted around five years.

It wasn't all fun - at 18, he contracted dysentery on the road to Marrakech and was deported - but Jansch remains misty-eyed about the innocence of those years. "It was romantic," he says, "because you had no fear of anything, because you didn't know. The world was there, and you just went out and found out all about it."

He suddenly falls silent.

"I think basically it was because I'd never known my father. There was no father figure ever in my household. By the time I was 16, my brother did his National Service, and he signed on for 12 years after that. Consequently he was never there. There was just my mother and my sister, and that was it. There was no guidance at all."

HE never met his father, who separated from his mother just after Jansch was born. "I probably saw him once. Actually, my mother wouldn't allow me to see him if he came round to the house. So I never heard of him."

Why did his parents split? "I've no idea. And it's always been taboo to speak of it." Was he curious? "Yeah. I've written songs about him. But he's never made any attempt to approach me, or - as far as I know - the rest of the family, in all that time."

Jansch, who is 64, now believes his father is dead. "I met a girl in a gig in Newcastle, and she came up and said that she'd just moved into this flat. A week after she was in there, the man upstairs died, and he had exactly the same name as me. So I figure it's got to be him. Janschs are quite common in Scotland, but one with exactly the same name "

Still, he seems curious about his father's life. "I am. But even now my sister doesn't want to talk about him. And my mother's dead now, so "

I ask whether his father's absence made him into the man he is, and he answers quickly: "No, I don't think so. Unless it's in the genes."

Jansch had his own brush with mortality three years ago, when he had serious heart trouble. Certainly, he looks far healthier today than when I met him last, just before his 60th birthday. "Previously, I wouldn't have known if I'd had a heart attack or not. You just don't know - it could be indigestion - you've got no idea. Then one day I was running for a bus; I was carrying this gear, and I got on the bus, and I thought I was going to pass out. When I went to the doctor's two or three days after that, I was telling her the symptoms, and she said, Well, you've got angina.' I didn't know what angina was.

"I went for an angiogram in the hospital, and they said, You need angioplasty. We'll take you in about two or three months.' I was set to go in for this routine operation. Well, I had a pain the day before. And that was it, I was in hospital and I didn't come out till two months later. They put me in for the operation and halfway through they said, No you have to have a bypass'. Four arteries had to be replaced. It was two days before the big bombs in London. So I woke up to all this news of the bombs, and the hospital was right next door to the Edgware Road. I couldn't understand what was going on."

I tell him he looks far healthier and happier than before. "Yeah," he says. "I now know that I was quite lucky. I could have had a stroke. It's terrifying."

RECENTLY, Jansch's musical efforts have been focused on Pentangle, who reunited for the 2007 Folk Awards ("the rehearsals were great. The show was a bit iffy") and will be touring in the new year. "At the moment we're just trying to remember all the songs from those days. Some of them I've never played."

In their absence, Pentangle's reputation has blossomed. Banhart's interest in Jansch began with Pentangle, and the group's eclecticism has led to them being described as the Grateful Dead of folk (a compliment, apparently).

Jansch says Pentangle's fusion of styles was organic. "Somebody would say, Let's do a Staple Singers song, and off we went. Instrumental numbers were a combination of what we'd been listening to: Ravi Shankar or whatever." The common link was jazz. "Danny Thompson and Terry Cox were the resident rhythm section at Ronnie Scott's and also they were in Alexis Korner's band. It was John Renbourn who introduced the rest of us to them. But long before I ever met them I was well into - through Davey Graham - stuff like Charlie Mingus. And Jacqui McShee's favourite was Miles Davis."

Jansch's recent solo album, The Black Swan, is a beautifully mature work, with contributions from Banhart, and guest vocals from Beth Orton. Oddly, Jansch is doing the same as he always has, yet his work sounds both timeless and contemporary. What keeps him interested in the guitar?

"Someone I love personally is Clive Palmer of the Incredible String Band who I've known since I was 16. His music is peculiar because if you close your eyes while listening to it, you can pitch yourself back 200 years. It has that much atmosphere."

He tells me about a show of Palmer's he saw at the 12 Bar Club in London. "He started slowly. When you start listening to Clive your attention's elsewhere. Eventually you get round to him singing, and by the third number in, you're captivated. Then he starts bringing people up. By the time the lights go down the stage is full of people, they're all singing, and the whole place is rocking like hell."

Surprisingly, Jansch witnessed much the same effect when he watched Pete Doherty at Hackney Empire, when the former Libertine invited his old bandmate Carl Barat onstage. "That started off fairly slow, until the audience started shouting out. The place was packed, and these young kids knew every single word. Literally. It was exactly like a folk club. Then when Carl Barat got up, that was just wild. To me, it was like music hall."

When Jansch played with Doherty he did him the honour of dusting down Needle Of Death, a song written for his friend Buck Polly, who died of a heroin overdose. Jansch doesn't like to perform the song ("it depresses me"), but was happy to play the guitar while Doherty sang. He was aware of the ironies. "It's normally addicts from the audience who shout out for it."

It all seems a lifetime away from those all-nighters in the Soho folk clubs. But Jansch can be content that his quiet genius is still being recognised.

He has a word for musicians he is not interested in - he calls them "slotted", meaning trapped by genre - but he is open-minded about people who are open-minded. That attitude had carried him through four decades. "If you were interested in music at all, it didn't matter where it came from. If you had a curious mind you'd obviously want to know about all music."

Bert Jansch and Espers with Eliza Carthy play Glasgow Royal Concert Hall for Celtic Connections on January 23