Serious documentary maker? Is Keith Allen having a laugh?

Tea with Keith Allen. Sounds like a laugh - two hours of Earl Grey and indiscreet chat with the chain-smoking court jester of the Britpop movement. Marvellous. Or as people used to say in what one might call his 1990s heyday: sorted.

Allen is one of those people who like to keep you guessing. One day he's an artist, the next he's an actor. Then he'll be making pop records, or popping up in another famous person's anecdote, usually involving late nights at the Groucho Club. He's impossible to pigeonhole, which is one of the many things that make him seem impossibly interesting.

If a social historian were to chronicle the development of the modern celebrity, this man would be everywhere; at least in the footnotes. Allen may never have achieved the commercial success of Damien Hirst (an old chum), or the rock stardom of, say, Alex James (ditto). Instead, his role has been to inspire, provoke, and bear witness to their circle's most notorious "moments".

Allen it was who, in the mid-Nineties, persuaded Hirst to serve up his penis, on a plate, to Stephen Fry, after the latter had requested a sausage breakfast at the Groucho. He's also, to take another example at random, the chap who publicly recalled making love twice to Janet Street-Porter - once on a pool table, the second time at her flat, where the "earth moved" (if only because JSP's bed was on wheels).

Along the way, he's written screenplays and pop songs, had a brief career as a stand-up comedian (it ended after he was knocked out by a soldier offended by one of his jokes) and recently published a revelatory autobiography. He's been sent to prison twice, and appeared in such iconic British films as Shallow Grave, Twin Town and 24 Hour Party People.

Most importantly, he's always boasted an unerring ability to draw attention to himself. Only last weekend, the Sunday papers plumped on a minor scoop in his recently released memoir Grow Up: the revelation that Damien Hirst once persuaded Sir Trevor Nunn to pay £27,000 for a "spot painting" that was actually the work of Hirst's two- year-old son Connor and Allen's 10-year-old, Alfie.

These days, Keith is mostly in the papers for being the father of Lily Allen, the troubled but supremely talented young pop singer. She has inherited his outspoken manner, together with his dislike for the fawning accoutrements of some other celebrities, and his appetite for occasional hellraising.

He's also been getting attention for his role in the BBC's Robin Hood as a delightfully wicked Sheriff of Nottingham, and is currently making a series of TV documentaries for Channel 4, which is the reason I've been invited to share his sofa at the Covent Garden Hotel.

Allen's latest documentary, which is screened tonight, is a fly-on-the-wall look at a group of American religious extremists. Though comic in tone, its underlying seriousness suggests that there may be slightly more to its maker than the wise-cracking wide boy we've come to know and love.

The programme has been called Keith Allen Will Burn in Hell, and follows him on a long weekend with the God-fearing congregation of Westboro Baptist Church in Topeka, Kansas, whose beliefs are based on the simple premise that, to quote their slogan: "God Hates Fags."

Keith Allen, of course, is not a "fag". Quite the reverse: he's famously heterosexual, and is said to have fathered a total of eight children, by six different women, two of whom have been his wives. In the eyes of the Bible-bashing American subjects of his film, however, his notorious promiscuity means he'll one day burn in Hell. Needless to say, it makes for high-octane telly.

We begin our interview with a moment of high cliché. Allen has run out of cigarettes, and no one in the room has a spare. So he grabs a half-smoked Marlboro from an overflowing ashtray on the next table, pats down the pockets of his jeans for a lighter, and sparks up.

Allen has a contract with Channel 4 to make films about what he calls "unusual people". Keith Allen Will Burn in Hell is the fifth of these - others have been on people like Mohamed Al Fayed, or the lottery "lout" Michael Carroll - and seems timely in light of both current affairs and the fashion for atheism, preached by the likes of Richard Dawkins and Christopher Hitchens.

"Religion is right on the agenda, without a shadow of a doubt," Allen says. "Historically, I suppose it always is at times of uncertainty. Usually uncertainty is defined by war, and that seems to be the case at the minute."

Westboro's congregation lives according to a literal interpretation of the Old Testament. As the USA tolerates gays, the church's bête noir, members campaign under the slogan "God Hates America," which most controversially means picketing the funerals of fallen US soldiers, waving gaudy placards.

It's a bit of a freakshow, but manna from Heaven for any deadpan documentary-maker. Indeed, Louis Theroux also recently made a programme on Westboro, though that took him several months, whereas Allen was filming for only a matter of days.

"I film documentaries over an average of four days, at the most. That's it, to get all the raw material. Then I leave them alone. Victor [Lewis-Smith] writes the commentary, John [Warburton] edits it, then we'll talk, he'll send me a DVD and I'll make my comments. It's a great way of working."

Some might call this method slapdash, but Allen prefers the big-bang approach. In Kansas, he provokes a semi-confrontational denouement after exposing one of the church's leading lights for having produced an illegitimate son.

One of the many reasons Allen made the documentary was to explore his own atheism. Unlike most non-believers, he claims, in all seriousness, to have once seen God. It was at Glastonbury during the 1980s, and (as is the case with most of the splendid anecdotes that litter his conversation), it involved enough mind-altering substances to stun a baby elephant.

"I'd taken something called DMT," he recalls, over the first of his many cigarettes. "Within two seconds, you're gone. Absolutely gone. And then I saw God, or I saw what I think is God, which is a sort of mathematical equation. I could suddenly work out things at the most astonishing speed to do with the symmetry of the world around me. It re-affirmed my suspicion that God is actually a collective entity of everyone's being."

In person, as well as on film, Allen's manner can be gruff, yet the showman in him is able to describe the 70 or so God-fearing American subjects of his television documentary with a degree of grudging respect.

"There's a part of me that does admire them, believe it or not," he says. "I admire the way they've managed to highlight what the church used to mean. Forget the homosexuality, which I think is a real problem for them; forget that, and they are basically attacking lazy religious observation.

"Their argument is perfectly well aimed at today's Church of England, for example, where there is such a lack of passion, and where it is often not about belief, but a chore. It's religion as a chore in this country: people feel they ought to go to church because they're kind of Christians. That infuriates me."

Allen's own lack of religious belief stems from a childhood divided between South Wales, where he was born 54 years ago, and Malta, where his tough-nut father was posted as a submariner. A difficult teenager, he was sent to borstal for burglary, where he was coerced into sitting O-levels, before discovering drama.

Despite being at best a moderately successful actor - he often gets recognised as "the dead bloke" from the start of Shallow Grave - Allen developed a real talent during the mid-Eighties for standing out from the crowd. He tried bits of stand-up comedy, and formed the pop group Fat Les, whose song "Vindaloo" became England's unofficial 1998 World Cup anthem.

Outside of showbusiness, Allen has always been portrayed as an overgrown teenager, into booze and shagging. His first wife, Alison Owen, produced the famous Lily, and her increasingly famous younger brother, Alfie, who is currently making a film in South Africa with Daniel Craig.

"I spoke to Lily this morning, actually," he says, when I bring up his daughter. "She's completely exhausted. You've got to remember that she's on her own. It's tough. It's really hard, that industry. People don't realise how fucking mind-numbing it is. It's a great thing to do for a bit. If you're good at it, stay with it. If not, do something else."

The flip side of fame, which Allen claims to have much experience of, is dealing with the attentions of the press. Though he enjoys fame and has often courted notoriety, he complains that the tabloids routinely make stuff up about him and Lily. "Most of the time, I don't give a fuck, because it's so obvious that they are just lying. For example, I read in the newspaper the other day - and this is absolute shit - that I comforted Lily at Princess Diana's funeral, when she was crying.

"They just make it up. In fact, I was at that funeral with Lily's brother Alfie. We were on the Mall, but the crowds were 10 deep, so we walked to the Groucho and nicked two stepladders out of the caretaker's cupboard. We walked back to the Mall and sold them for $500 each to some Americans to sit on and take photos. That was me going to Princess Di's funeral."

Might Lily now do something else? Recent reports have suggested that she might be about to jack in pop for a career in acting. Allen has a vaguely casual plan to offer her a part in a new TV series he is working on.

"There's this idea for a series I had six or seven years ago. One of my ex-wives sat on it, and unbeknownst to me recently got someone to write up the synopsis again, and I've just discovered that the BBC have put it into development. It's called Karaoke Wars, and is set in Wales. It's a drama, and if Lily gets bored I might put her in it."

Exes crop up regularly in Allen's conversation, partly because he attempts to stay on friendly terms with most of them. His autobiograpy was even half-ghosted by one. "I didn't want to write the book, but they offered me a shedload of money and I was seduced into doing it. Originally I thought it would work great, because I would just sit there while Zoe, my girlfriend, wrote it all out. But then it all had to change, because although I started with her writing it for me, I finished the book going out with someone else, who'd had my baby. It was a massive sea-change in who I am."

Today, he lives in the heart of the Gloucestershire celebrity belt with his girlfriend Tamzin Malleson, a former co-star 20 years his junior, and their child. He does middle-class things like playing golf, and only rarely ventures up to London, saying: "I'm quite capable of getting pissed in the country."

Professionally, he claims not to make a vast amount of money from his career because he "doesn't do" voiceovers or TV adverts on principle, and looks firmly down his nose at the Jamie Olivers or John Cleeses who have sold out to become the faces of the supermarket industry. "Most of the things you're being asked to sell, I have a problem with. Like those people who do supermarket voiceovers. I don't see how they can live with themselves. I would love to see all my contemporaries whose voices I hear doing those commercials suddenly say, 'We've made £2m, and now we're going to make an advert saying Tesco is shit.' But that ain't going to happen."

To keep the wolf from the door, Allen is currently sticking with Robin Hood, which is filmed in Eastern Europe, and making further documentaries. One potential future subject is paedophilia, something he considers to be a misunderstood topic as "95 per cent of paedophiles don't go near children, because of what they know they'll do if they do".

It's a bold move, considering that the last programme-maker who adopted a vaguely satirical touch when dealing with paedophilia was Chris Morris, whose Brass Eye on the subject prompted a national outcry.

But Allen is used to public disapproval, particularly as regards his attitude towards the institution of marriage, which has been the subject of several sniffy newspaper profiles. He now says he'll never marry again, though he has few regrets about the effect his failed relationships might have had on his children.

"It did hurt the kids, but we came through it. It's better not to bring up kids in an unhappy marriage. But it's painful, splitting up. It is painful. I'm not cheap about it: it's the most painful thing, and it can be horrendous."

Like any considered atheist, particularly one who will burn in Hell, he lives according to a moral code that refuses to romanticise things like love, or devotion. It all helps, I suppose, with his je ne regrette rien approach to life. "It could be proved that love is actually a chemical state. So get on with it. Do you know what I mean? Don't fucking write books about how you are supposed to stay in love; just experience the lot and then get on with your life. I cannot bear the idea of living my life on somebody else's terms."

That line rather sums up Keith Allen. Some might read it and think him a selfish man. Others might say it makes him a realist. Either way, he adds gaiety to public life - and he makes a pretty decent documentary.

'Keith Allen Will Burn in Hell', Channel 4, tonight at 10.30