Look past the barnet and Boris Johnson's ambition is more than just blonde

Well, who'd have thought it? Beneath Boris Johnson's bumbling buffoonery, the carefully-constructed casual appearance, and the studied self-mockery, there beats a heart of pure ambition.

According to his sister, the voluble Rachel, he is an intensely driven individual. “He sees life as a competition,” she says, “and he always wants to be top.” Michael Cockerell, the acclaimed film-maker whose revealing portrait of the Mayor of London has loosed a 48-hour Boris-fest on assorted media - beginning with Eddie Mair's thrilling interview on Sunday morning - told me about one of the off-cuts from his documentary which exposed the true extent of his subject's competitiveness.

There is a scene in which Boris is playing tennis. “He is rather good,” said Cockerell, “even though he plays with a wooden racket.” At one point, Cockerell - no mean player himself - had to stand in for a game of doubles. “When I was at the net, Boris was hitting the ball so hard at me, it felt like he was trying to take my head off,” explains Cockerell. “But I did get the ball back a few times.” Afterwards, Boris took Cockerell aside and said, less than half jokingly: “I'm worried about this film. I think you might cut it so that it looks like you're a better player than I am.”

The picture built up by the programme is one in which sibling rivalry plays a significant part in the shaping of a “formidable” (Ken Livingstone's word) politician. His mother says that when his younger sister came along, the look on Boris's face was one of panic and fear. Does he regard David Cameron - a contemporary at Eton and a political bedfellow - as a brother?

If so, it might explain his desire to get one over on the Prime Minister. Again, Cockerell shared with me an insight that didn't make the film. Mr Cameron is accompanying the Mayor up the stairs at No 10, and they are looking at the pictures of former Prime Ministers, of whom about a third went to Eton. At one, Boris says “Scholar” - he was a scholar, and Mr Cameron was not - and at another, he says “Pop” - this denotes the elite Etonian society, of which he was a member and Mr Cameron was not. Subtle, almost invisible to the eye, but effective put-downs.

I have always found it difficult to believe that anything Boris Johnson does or says is as extemporised (to borrow his lexicon) as he would have us think. It's not beyond the bounds of possibility that he has a team of expert touslers who come in every morning to make sure his barnet is in perfectly haphazard shape.

There is a moment in the Eddie Mair interview, as Boris is being asked about his affair with Petronella Wyatt, when the mask slips. It's worth watching carefully. Mair says he can understand why Boris doesn't want to talk about it. “Why should I?” he snaps, and for a nano-second he loses it, and the real competitor emerges, not quite so cuddly after all. I don't think we need to spend too much time wondering whether Johnson, B really wants to be PM.