Whatever people say about Keith Allen as a person, he is always a powerful and compelling screen presence. Dangerous. Threatening. Sexy in a masterful, I'll-have-you-whether-you-like-it-or-not sort of way.

"It's all in the eyes," he announces, with a sidelong look of such menace it quite puts the wind up me, just as he probably intended. He then says that when he was up for the part of Jonas Chuzzlewit - thug, wife-beater, all-round nasty bit of work - in the BBC's recent production of Martin Chuzzlewit, and was asked why he should be cast, he said: "Look into my eyes. Now tell me I am not Jonas Chuzzlewit." Needless to say, he got the part there and then.

He is not, however, an actor who bothers with technique. "I hate technically constructed actors. In fact, I hate anything technically constructed, unless it's a bridge." If he were asked to play Napoleon, he would not be Napoleon. He would, he says, be Keith Allen as Napoleon. I think he means that he is the actor he is because of the compelling, powerful person he is. His own character colours every role.

On screen, this combination always works marvellously. His Jonas was so wickedly wonderful he quite stole Martin Chuzzlewit from the likes of Sir John Mills and Sir Paul Schofield. His recent drug-crazed cameos in the films Trainspotting and Shallow Grave are unforgettable. ("Although less of the `cameo', please Deborah ..."). He can currently be seen as Byron, the wholly egocentric car dealer caught between wife and mistress, in Debbie Horsfield's comedy-drama Born to Run on Sunday evenings on BBC1. He is, needless to say, compulsively watchable in this, too.

So, on screen we are talking hot stuff. But what about off it? I'd heard he can be mean and cruel, especially if you bore him or ask him stupid questions which might not, in fact, be stupid, but he can make them seem so. He doesn't ever feel the need to explain himself. He may well walk in, say "sod this", so the story goes, and walk out again. He doesn't care what you may make of him, because he doesn't give two hoots for anyone.

Mr Allen, 43, was expelled from pretty much every school he went to. He has done spells in borstal (petty theft, cat burglary) and Pentonville prison (smashing up a nightclub). He has five children by four different women. He has nothing to do with three of these kids because they resulted from one-night stands, more or less, so they aren't his responsibility, are they? He is continually being chased through the courts for child maintenance by someone or other.

He is also the stagehand who, in 1975, strode out naked on to the stage at the Victoria Palace Theatre as Max Bygraves was mid-way through his routine. Yes, this was very much a premeditated act. He had not been much impressed by Max. "He was earning God knows how much, but when he threw a Christmas party for the staff he had us bussed to one of his hotels in Bournemouth where we had to pay for our own drinks. The tightarse!" Yes, he was instantly dismissed. But it was very much worth it, he says. I do not think you could ever describe Keith Allen as a forgiving sort. Should you ever chance upon him down the Groucho Club, I would stand your own round, if I were you.

Although Keith has a house in Gospel Oak, north London, which he shares with his current girlfriend, TV producer Nira Park, he is known to spend most of his time in the Groucho, a Soho drinking club, where he consumes monster amounts with the the likes of Alex from Blur, the Gallagher brothers, and his best friend, Damien Hirst. He is godfather to Damien's child. Damien, he tells you approvingly, has just refused to take part in this British Airways redesign business because "he wanted Concorde. And they wouldn't give him Concorde".

However, to meet him I had to travel to the picturesque Sussex village of Seddlescombe where he is holed up in, incongruously enough, the sweetest of thatched, 16th-century cottages. This is because his TV production company, Big Talk Productions, has just been commissioned to write a drama- comedy series for Channel 4 and he and his five co-writers need to get the job done. Mr Allen started out as writer-cum-comic rather than actor. His comic heroes? Ted Ray, Les Dawson and Freddie Starr. And Lenny Henry, I ask, because I'm interested to know if anyone on this earth finds him funny. "You can write down Keith Allen says Lenny Henry is about as funny as a heart attack," he says.

Anyway, his new series, if I understand it correctly, is about a community beset by strange magnetic forces. The main characters are Rita and Sheldon Cohen, Jewish converts who run a Welsh tapas bar and who get very cross every time the magnetism causes the milk and meat cutlery to fly about and get all mixed up. I think it would be safe to say that wherever Mr Allen goes, he takes his own world with him.

Surprisingly, he greets me warmly. "Come in, come in," he beckons enthusiastically from one of the open, mullioned windows. (Oh God, he's not drunk already is he? No, he insists, he is not. Some days he doesn't drink at all, and this is going to be one of them.) He makes lunch - a pot of tea, french bread, posh lettuce - and serves it in the garden. Although he initially insisted he could only spare one hour, he talks for two.

To be honest, it's a bit disappointing. It's like getting all worked up about a supposedly scary fairground ride only to find it is actually a rather kittenish event. I sense he answers questions as truthfully as he can. The more intimate they are, the more he seems to relish it. So, Keith, when did you lose your virginity? "At 11. To a 12-year-old on board a disused boat moored outside Broomfield Park Comprehensive." Did you go on to date her? "God, no," he gasps. "Two years later her mother offered herself to me." The implication here, I think, is that the family was a bit common.

Perhaps he just isn't in a frightening mood today. Or perhaps he has mellowed? He says not. He says he's never been as bad as painted. "I am very domesticated and, Deborah, do a lot of washing up," he adds. Cooking? "I do a very good Sunday roast." DIY? "Now, Deborah," he says with the menacing sidelong look, "you are being silly. Why would anyone want to do DIY?"

He was born in Wales to a Navy submariner and a one-time waitress. His father was away for large chunks of his childhood. Then, when Keith was 12, he was posted to Singapore. His mother went to join him, as did his two younger siblings - Kevin, now a filmmaker, and Susan, who looks after animals - while Keith was despatched to a public boarding school here. He only saw them once in two years. Was getting into trouble as a kid his way of focusing his parents' attention on him? He insists not. "I can't even remember ever missing them."

He first got into major trouble when he was 11 and was caught nicking tennis balls from Woolworth's. The police came round to his house, something he did not take lightly.

"A panda car outside, for Chrissakes! My parents weren't in the first time they came. I'll come down to the station with you. I'll do anything. Just don't come back here again, I told them. They said they would be coming back because they wanted to talk to my dad.

"They didn't return for three days, during which time I lost half a stone. My father's wrath was terrible. And I was punished by not being allowed to watch the World Cup Final. I had to stay in my bedroom. At some point in the game - and I remember this clearly - my dad came upstairs for a pee.

"I opened my bedroom door a chink. "2-1!" he exclaimed excitedly. But he then remembered what I had done and shouted, "BACK IN YOUR ROOM".

Yes, he says, he did worry about the distress he caused his parents. Whenever his mother cried over him, as she often did, "I felt terrible shame and guilt." But it wasn't enough to stop him behaving in such a delinquent way. "There must have been gratification in it somewhere, I suppose. But mostly it just alienated me from everyone. I never worked in a gang or anything."

He went to a comprehensive then to a public boarding school, from which he was expelled for "the usual shit ... drinking, smoking, throwing fireworks from the dormitory windows, generally leading the other boys astray". He ended up in borstal because he had turned his hand to cat burglary, but was a lousy cat burglar. "I was good at getting in, but rubbish at getting out." He did not mind being in borstal. "I got to play a lot of football." Later, when he did a five-week stretch in Pentonville, he didn't mind that too much, either, although his cellmate was something of a pain. "He was a Scottish drone who went on and on about all the bank robberies he'd done, whereas everyone knew he'd only ever stolen bags of sweets."

Is he scared of anything? Yes, he says, he is scared of being found out. Found out as what? "As someone who is inconsequential, thick and daft." So that's how, deep down, you think of yourself then? "Nope. I am saying that sometimes I think these things. And sometimes these thoughts predominate." But not very often, I imagine. He was a rubbish cat burglar "because my heart wasn't in it. If it had been in it, then I'd have been good". Does he see himself as sexy? "Yes! Yes!" How does he rate himself as an actor? "I'm good. Bloody good. The best."

He thinks he became an actor because, with all the lies he had to tell, he was pretty much an actor all along. But it didn't crystallise for him until he was 19 and briefly went to a Welsh drama school before coming to London, first as a stagehand then as one of the Comedy Store's major players. He jacked this in for straight drama because, he says, he simply got bored.

I wonder what Keith Allen really cares about? "My children," he says without hesitation. "Although I know that is going to sound weird because I don't even know all of them." He says Child One, Two and Three were not his fault because he only slept with the mothers once. (Actually, with one of them it was three times, but she got pregnant on the first occasion.) But Keith, I say, have you never heard of contraception?

"But one of the women was 35, for Chrissakes. She wasn't a kid." In so far as Keith ever explains himself, this is probably as good as you are going to get.

He says he is not inquisitive about these children. "They have their own lives." Yes, one day they will probably come knocking at his door. And? "I'll be ready." In what way? "I'll be ready to talk to them, acknowledge them, do what they want." It does not occur to him that it might be too late by then. That any damage will already have been done.

However, he says he does care very much about Alfie, 10, and Lily, 12, his two children by his ex-wife, Alison, who eventually left him for Harry Enfield. "Yes, of course I still speak to Harry. I never had a problem with it." He speaks to them almost daily, he continues, and ends every call with "I love you, love, love you." No, his own parents never told him they loved him. But then parents of that generation just didn't ever say such things. Plus, he says, his mother is a wonderfully old-fashioned sort.

When he married, she drew Alison aside in the vestry to say: "You will keep him clean, won't you dear?"

So, what are we to make of Keith Allen? As a bloke, I'm not so sure. One minute I think he's a wholly irresistible force. The next, I think he's a bit of an S-H-I-T, as Alan Clark's wife, Jane, would say. But it is this mixture which, on screen, gives him such star quality. As an actor, he sure brings a lot to the party. May even be the party, most times.