Feel free to use almost any adjective you like to describe Peter Mandelson: with the exception of "fat" and "bald", they more or less all apply. He is both delightful and appalling; fiercely loyal but calculatedly indiscreet; jovial but venomous; cold but flirtatious. One minute, he'll be soothingly placid; the next, he'll be whipping himself into a frenzy of petulance or anger.
The backdrop to Hannah Rothschild's Storyville documentary about the Prince of Darkness (23 November, 9pm) was exceedingly boring: drab offices, the back seat of his car, endless corridors. Even his home looked drably chintzy (I was expecting mid-century modernism). Mandelson himself, however, was mesmerising. "I know everything," he said as the film opened, which gave me the shivers. But even when he was silent - and he uses silence the way other people use a cosh - I was spellbound. I could watch him eat yoghurt all day; a stoat devours a field mouse with less menace.
He also has a fascinatingly peculiar gait - he walks as if on tiptoe - which makes him look as if he is trying to avoid stepping in a dog turd, even when indoors. This is a rather good metaphor for his career.
Why had he agreed to this film, in which he appeared in his leather carpet slippers, looking like something left over from Man About the House? You will say that he knew exactly what he was doing. Labour was going to lose the election no matter what and the film would appear only after it. The project was yet another knight's move on his part and a symptom of his depthless vanity.
But I think you'd be wrong. I interviewed Mandelson on a day Rothschild was filming - my nose and teeth appear 18 minutes in - and it was my strong impression that he had underestimated her. It takes one to know one and I could all but smell her beadiness, her patience and her willingness to be invisible, inconvenienced and mildly patronised for the sake of nailing her quarry.
Did she nail him? Sort of. As he prepared to leave the Department for Business, his staff gathered for a small party. An extremely frou-frou bit of patisserie was on the table, as laden with fruit as a dowager duchess's hat. Mandy made to cut it. "Is there any cake underneath all this?" he asked.
If the laughter in the room sounded hollow, perhaps that's because everyone was wondering if he was making a point - I know I was. What were the past 13 years all about? We, too, were promised cake, but . . . well, you know the rest. It's still forbidden to put the word "camp" next to Mandelson's name. But this is exactly what he is. And he knows it: he utilises it with exquisite precision. We saw it in his baiting of George Osborne, whom he spotted after a leadership debate with only one sad little journalist to talk to (a football crowd trailed Mandy); and in his complaint that, when it comes to Gordon's dishevelled appearance, he would "settle for the tie being straight"; and in his account of his morning routine, which left him a "little ragged" (the routine in question involved a personal trainer, a search for cufflinks and muesli). Other jokes were more unwitting. Tony Blair is "a nice person". Gordon was "absolutely brilliant" in the leadership debates. He doesn't care how mean the Times leader is about him: he just wants to "understand" it. I also loved the delightful footage in which Mandelson made salad dressing in an extremely Old Labour earthenware jug.
Sincerity? Yes, I grant you this is another word that really doesn't apply. Late at night, just him and Hannah and a crackling fire, he arched his reedy neck and threw back his head and you suddenly imagined - steady! - that he wanted to be loved or, at least, that he truly wanted to talk, to expunge a little of the old hurt. He reminded me forcefully of a sleepy greyhound, on its back, legs in the air, waiting for its tummy to be tickled. But thensomething - his 6am wake-up call, his red boxes, his vampiric taste for the blood of career politicians - would bring him back to himself and he'dstalk off to bed. The vinegary goodnight smile told you that you'd been had. On top of everything else, the man's an incorrigible tease.