SERGEANT DAVID Baxter, a tall, bearded gunner from Glasgow, was describing life in the forward operating base (FOB) - nicknamed "Incoming" - when the machine gun fire started. It was the third Taliban attack of the day. The noise was a few hundred yards off with no rounds whipping overhead, so even though he was standing out in the open, albeit inside the base's perimeter, Baxter hardly batted an eyelid. Instead, he just muttered something about the Afghan army soldiers shooting at stray dogs. Then the crump of mortars started, much closer this time, and the siren to take cover went off while the gun battle at the Afghan army's position rose to an angry crescendo.

The bunker, a 50-yard sprint across open ground, was full of laughing gunners pulling on body armour and helmets after their lazy afternoon under a winter sun had been rudely interrupted.

"Welcome to Incoming," Baxter grinned as a couple of Royal Marines sprinted into the bunker and crashed into its occupants. At a trestle table, young women soldiers spoke urgently into radios. Scruffy marines joked about "Terry" Taliban - the British soldier's half-affectionate nickname for his enemy. They reckon Terry is a pretty hopeless soldier, returning again and again to the same firing positions where he is routinely killed. Nor is he very good at handling his weapons or planning his attacks. But he is ballsy, and the young men of Incoming respect him for that. It is a uniquely British moniker, like something cooked up by Viz magazine, and a subversive echo of the Vietnam war's "Charlie" that only the British soldier's cheerful sense of subversive black humour could have come up with.

A few weeks earlier, the Taliban mortar crews had managed to hit inside FOB Incoming, but their aim was wild today. No bombs landed within the towering perimeter walls constructed from massive plastic containers full of dirt. Most of the rounds seemed to be outgoing, fired from the marines' own mortar pits.

Chatting idly in the bunker while the battle raged outside, the British soldiers explained that they had recently shot a number of Taliban out in the fields where the fire was coming from. Since then the accurate mortar fire had ended. They revealed - with some satisfaction - their theory that they had got the Taliban's mortar crews.

Most of the soldiers were in their 20s or younger, with accents from the poorer parts of the north of England and Wales predominating. They didn't look conventionally military, with a profusion of unkempt beards, sideburns, and scraggly moustaches, which they are allowed to grow on the FOBs but must shave off when they go back to the main base.

The FOB is one of a couple of dozen the British have constructed along the Helmand river at strategic points, looking like the kind of outposts once held by the Foreign Legion. The MoD does not want the Taliban to know the real name of the FOB, so the soldiers - who face more daily mortar fire and machine gun attacks than almost anywhere else in Helmand - nicknamed it Incoming.

Marine Simon Vaughan, from Newport in south Wales, said: "It's pretty tough here. It's cold at night, the living conditions are basic, and the Taliban attack every day. But you join up hoping to fight a war like this. Nobody here wants to be anywhere else."

The soldiers serving in Incoming spend much of their downtime swapping stories. Lance-Corporal Kearan Varley said: "The atmosphere here is so blasé it is unreal. They hardly flinch when mortars come in. At lunchtime on the day I arrived the Taliban launched a big attack and Mitch, the cook, ran out of the kitchen with his gun and went up on the parapet, blazing away still wearing his apron. Where else would you see something like that?"

Like the British military in any campaign, Incoming has its mix of characters and not all of them British. A giant Fijian refueller joked about the cold. At home before joining the army he had never seen snow or imagined what -10˚C at night was like. A small detachment of Gurkha engineers were busy every day extending and repairing the FOB's fortifications, which would have presented a pretty respectable challenge to a besieging army in the Middle Ages.

They had taken over a half-derelict Afghan building and turned it into a little corner of Nepal, complete with national flag, a DVD player with a pile of Bollywood movies, and plenty of milky chai. Afghanistan, with its insurgents, brigands, and dangers, reminded them of home with its Maoist revolutionaries and bitter political problems.

For the British troops, living conditions are rudimentary. The grub is plentiful but basic. Entertainment consists of a giant screen and DVD player in a tent where Steven Seagal movies play endlessly. Hours and hours of duty consist of staring into the dark looking for the Taliban or manning the parapet with a machine gun waiting for the guerrillas to open fire.

The FOB was deliberately set up as a Taliban magnet. On one side is a desert across which British supply convoys can cautiously travel. On the other are poppy fields, trees and deserted villages leading down to the "green zone" of dense vegetation along the Helmand river where the Taliban move around and where the attacks come from.

The FOB controls an approach to the vital town of Sangin. Many of the roving jihadis who want to fight the British are drawn to it. Mostly their attacks are hurried and ineffective. The Royal Marines and soldiers are often fired at but rarely hit, and most of the casualties so far have been minor ones from shrapnel. Its defenders know the risk they are taking in Britain's most high-intensity conflict since Korea 50 years ago. On Christmas Eve at a nearby FOB a young marine ran over an anti-tank mine, losing both of his legs and an arm.

A nurse at Camp Bastion, the sprawling British base in the desert, described having to deal with such a terrible casualty. "The worst thing was thinking about his family, who had been looking forward to Christmas. Then they would have to deal with his injuries," she said. Body armour and battlefield surgery means casualties who would have been deaths in previous conflicts can survive in Helmand, but are often maimed.

Landmines are the biggest threat. The Taliban sneak up to the FOB approach roads and bury them at night, hoping the British will run over them on patrol.

The young soldiers and marines constantly head out of their bases to attack the Taliban and keep their enemy off balance. A few days after the mortar attack, a party of marines were chatting at 8pm before leaving Incoming on a night patrol to the green zone to hunt their enemy. Sitting on the cold ground in the dark with faces blackened and weapons ready, they joked together like a group of Boy Scouts preparing to head out on a lark. In the distance, unexplained flares occasionally lit up the horizon, while overhead mysterious aircraft and drones circled under a sky full of bright stars. The camp was almost pitch black. Lights are banned to make it harder for the enemy to aim mortars at night.

Hours after the patrol left, the dull thump of distant gunfire sounded from the green zone, the noise of the British rifles. The higher-pitched crack of the Taliban's AK-47s was not heard.

As well as fighting their own war in the fields around the FOB, the British are trying to win local hearts and minds as part of a counter-insurgency campaign. Major Adrian Morely, a bearded and cheerful Royal Marine who looked as if he had stepped out of a Victorian expeditionary force, said that, between defending the FOB by day and leading patrols by night, he was attempting to rebuild a bridge and get an irrigation scheme going for farmers, who are deeply suspicious of foreigners, especially foreign soldiers.

The British believe they are slowly prevailing against the Taliban, who they killed in large numbers last year and drove out of key positions, including the strategic town of Musa Qala to the north. But they think that only by winning over the population will they bring stability to Helmand and one day manage to extract themselves from the province.

Not that political matters are of much interest to the enlisted men here. Few of them seem to take much interest in why Britain is in Helmand or the rights and wrongs of the conflict. Where American soldiers will give a lecture about bringing democracy or fighting terrorism, most British soldiers will look uncomfortable and explain with a shrug that they are doing their jobs.

Some soldiers do question what Britain is doing in Afghanistan. "It costs a fortune for us to be here and we don't seem to be really achieving anything," one squaddie said. "When we leave in a few years' time, this place will simply go back to its usual state of mayhem."

Others believe stabilising Afghanistan is worthwhile, and necessary to defeat international terrorism. For most, it is a chance to do some real soldiering against an enemy who is prepared to stand up and fight. Veterans of Iraq enthuse about Afghanistan, a man's war compared with the urban terrorism of roadside bombs and hit-and-run mortar attacks they had to contend with in Basra.

War in Helmand tests everybody in Britain's 7000-strong force. And they know it could continue to do so for years or even decades to come.