So I had been expecting an encounter with a lairy, tanked- up version of Attila the Hun. What a surprise, then, a disappointment even, to meet someone so likable and, well, sober. To be sure, Allen has "a past". As a teenager, he was expelled from boarding school and landed up in Borstal for burglary. Later, he served five weeks in Pentonville prison for trashing a nightclub. He is said to have fathered five children by four different women.
His career, too, has been littered with "incidents". In 1975, he was sacked from his job as a stagehand at the Victoria Palace Theatre after walking on stage stark naked in a protest against the show's star, Max Bygraves. In the early 1980s, he threatened to throw darts into a comedy- club audience and directed a fire hose at a heckler.
The balding, powerfully built Allen is a natural-born live wire. During a two-hour interview and an hour-long taxi-ride afterwards, he constantly leapt about, delivering a wicked impersonation of EastEnders' Mike Reid one minute and singing his deeply unflattering new song about Chris Evans the next. But he's very far from being the bastard son of Ozzy Osbourne and Liam Gallagher depicted in some sections of the press. He contends that a reputation for "partying" is fiendishly difficult to shed.
"It comes with the territory," he claims. "If the press can take the easy option, they will. To them, I'm always `the wild man'. I used to be `the wild man of comedy', but `the wild man of character acting' hasn't quite got the same ring."
What bugs him most is the poetic licence he says the papers take with him. "I'm always mildly surprised to learn things about myself that I never knew. I didn't realise that I was a heroin addict, for instance, or that I had held a commissioning editor from C4 against a wall at knifepoint, or that I'd had an affair with Kate Moss. A lot of what journalists write is drivel. But people don't really care. They know it's all tomorrow's fish-and-chip paper."
While admitting he is naturally sociable and still has his moments - "Keith Allen in `No Vices' scandal would be a real headline" - the 45-year-old reckons he has now mellowed. "Your body clock ticks away, and you'd like to leave the planet with a bit more information than you had when you arrived."
As if to underline his increasing maturity, over the past few years he has been taking roles in ever more serious dramas - the ill-fated lodger in Shallow Grave, the memorably creepy Jonas in BBC2's Martin Chuzzlewit, the weak adulterer, Byron, in Born to Run for BBC1, and the all-too-plausible poisoner in ITV's The Life and Crimes of William Palmer. In perhaps his most challenging role yet, he is now playing Jack Denby, a maverick probation officer in Jack of Hearts, a new BBC Wales series which starts on BBC1 tonight.
This part is an opportunity to demonstrate his versatility - the moody, individualistic Jack is light years away from Allen's two most popular recent roles as Fat Les, the creator of the World Cup chant, "Vindaloo", and as the evil Tooth Fairy in a mouthwash commercial. It also highlights his sheer screen presence, described by one journalist as: "Dangerous. Threatening. Sexy in a masterful, I'll-have-you-whether-you-like-it-or- not sort of way." He commands attention even when - as in the first episode of Jack of Hearts - he's lying on his bed, staring into the middle-distance and contemplating why his girlfriend has deserted him.
"You can't learn how to have presence," Allen says. "I walk into a room and people say `I wonder where he's been?' When other people walk in, they say `I wonder where he's going?'"
Not a man short of self-confidence, he continues that "it comes down to one thing, your level of relaxation. Performing in front of cameras, a lot of actors feel intimidated. But I've opened for The Clash in front of 15,000 people. After that, standing in front of 30 crew members seems fairly easy."
Where Allen is not so relaxed is on the subject of TV commissioning editors. "So much TV is up its own arse. It's frustrating that a medium that can be so useful is actually the opposite, especially at prime time. They churn out all those endless, drivelly series. There's one about a badger detective now - what are they on about? If there's an endangered species, that series should be it. There must be writers out there who could give us another Our Friends in the North or Band of Gold, but they just force- feed us trash about detectives. We wrote a comic-strip spoof about a gourmet detective - `one murder and two recipes a show'. You're laughing, but then they actually made something similar called Pie in the Sky. We'll have a milkman detective next. Or how about an air hostess detective? The locations would be superb."
Sadly, Allen has no time to appear in such a series himself. Currently shooting Bob Martin, a Larry Sanders-style behind-the-scenes TV send-up for ITV, he is also writing a film called Three Lions in My Bed, about a pub football team which wins the FA Cup, considering a part in a movie about Coleridge, and planning a Fat Les millennium single with Alex James from Blur, and his old mate Damien Hirst: "We feel very strongly that we'll be taking on the Spice Girls and Robbie Williams."
But don't all these different activities leave him dazed and confused? Apparently not. "I like juggling," Allen says. "I'd get bored if I did just one thing. Anyway, all those boundaries are now being broken down. I exist in a world where I can be the Tooth Fairy, Fat Les and Jack Denby, and no one will raise an eyebrow. People's reference points aren't as specific as they used to be. I enjoy fluidity. I don't know what kind of actor I'd be if I was only an actor. My career plan is that I have no career plan."
Magnetic as they come, Allen is very much one of life's originals. According to him, "if everybody was a bank clerk, then I would be a rebel. But if everyone was a rebel, then I promise you I would go out of my way to be a bank clerk."
So doesn't starring in a high-profile BBC1 drama series seem just a touch mainstream? Might it even be construed as a sell-out? "It didn't do Robert Carlyle any harm when he did Hamish Macbeth," he argues. "There's no lack of credibility in being on BBC1."
"Anyway," Allen adds with a laugh, "I'll probably fall flat on my face in Jack of Hearts, and people will say, `it's shite, bring back the badger detective'. If that happened, I would have to open a restaurant."
Now that would be a pretty lively establishment...
`Jack of Hearts' begins on BBC1, tonight, 9.30pm