Sitting in a room upstairs at the Groucho Club in central London - the scene of some of his allegedly bacchanalian exploits - Allen at first does nothing to disabuse me of his reputation as a party animal able to paint the town scarlet without any help from Dulux. Rough of throat and chin, he admits to nursing a hangover the size of Soho. "I don't ever want to drink or smoke again," he croaks, before adding with a mischievous grin: "you haven't got any fags, have you?"
But later he simply laughs off his Oliver Reed-shaped tag. Brushing aside suggestions that there is no smoke without fire, he asserts: "People who work with me will tell you that I'm not like the image. I'm not an alcoholic or a drug addict. Those sort of stories come with the territory. I'm a fairly gregarious person, so whether I was doing this job or not, I'd still live in the public eye. Journalists have so much newsprint to fill, the details are the last of their considerations. They go to the graveyard to dig up the details. I'm just glad my parents have got used to this shit.
"I'm supposed to have had seven kids by five women - what?" he exclaims. "I was most interested to read that when I went on honeymoon to Mexico, it was snowing and there was a hurricane. Utter drivel."
Drivel or not, all this hellraiser baggage merely adds to the potency of casting Allen as the sublimely evil title role in ITV's glossy new period drama, The Life and Crimes of William Palmer. Based on a real-life case, this two-parter tells the story of an ostensibly respectable Victorian doctor who is in fact secretly poisoning enough members of his family to fill up the local morgue.
Lining up his irritating mother-in-law as a candidate for the marble slab, he tries to persuade her to move in with him so he can more easily bump her off. When she finally agrees, Palmer pats her on the shoulder and says with a sinister smile: "Well, that's settled then." He is all but sprouting horns and a tail.
"There was nothing appealing about William Palmer, I can assure you," Allen says. "He was very charming, charismatic and psychopathic. He got away with it for so long because of his class. On reflection, of course, people looked at the case and thought, `It was so obvious.' But people just didn't want to believe that a man in his position could be guilty."
Palmer merely follows in a long line of blinding baddies from Allen - think of his roles in Shallow Grave and Inspector Morse. He ventures that the former was "a film of its time". "There is a generation who like going to see things where there are no sympathetic characters. Look at the success of Seinfeld. When you see my character in Shallow Grave, you think, `Great, here's the lead,' and then he goes and dies."
But perhaps his most noteworthy nutter was Jonas in the BBC's Martin Chuzzlewit. For all his eye-catching evil, Allen was determined to make Jonas credible. "All actors will tell you that baddies are where the money is because they stick in people's minds," he says. "Jonas was a brilliant part. But I actually felt sorry for him because he was the product of his father - what you sow, so shall you reap. You've got to make baddies human. Look how normal Rose West seemed. Outright monsters are not plausible."
Allen may have started out on the stand-up comedy circuit, but he has taken on more and more serious roles in such work as The Homecoming on stage and Born to Run on television. Yet he still draws sustenance from his years in live comedy. "I remember Richard Eyre auditioning me for Murmuring Judges at the National Theatre," Allen recalls. "He said, `The Olivier is a big stage, how do you feel about that?' I replied that when you've opened for The Clash and The Stranglers in front of 15,000 people at Wembley, then 1,100 members of the bourgeoisie are not going to put the shits up you. That kind of confidence comes from comedy."
No friend of false modesty, Allen continues, "Unlike virtually everyone else, I know how to work a room. That comes in pretty handy. If you're not getting any laughs, then at least you can make people watch you."
That is the root of Allen's appeal - whether painting characters in light or shade, he makes them compulsive viewing. Indeed, whatever he's up to - striding around an upstairs room at the Groucho performing to an invited audience of one, appearing in an all-male production of Macbeth, hosting a breakfast show on pirate radio, featuring in England's World Cup song, or playing cricket - he has enough spare charisma to power the National Grid.
You still get the impression that the TV establishment view him as rather difficult, though. "Dangerous" is an epithet applied to him so frequently that it is now a cliche. "I have a focused career plan, which is not to have a focused career plan," he declares. "Clout doesn't mean shit. You're always at the whim of some bonehead somewhere."
Allen admits that ITV took a lot of persuading to award him the lead in William Palmer. "They had to argue for me to be ITV Network Centre-approved, as they put it. But once you are ITV Network Centre-approved," he says, "you can become that whistling detective on the moors for the next four years if you really want to."
Despite - or perhaps because of - his "attitude", Allen is never going to go short of work. "People need actors my age," he affirms. "There are loads of kids, but there aren't many older actors who are any good. They fall by the wayside and open knick-knack stores instead." Somehow I can't see Allen following their example.
`The Life and Crimes of William Palmer' begins tomorrow at 8.30pm on ITV.