Another  MP pondering the possibility can now envisage him becoming Tory leader, perhaps: "He has flaws and people like that they are obvious. They see that he isn't smooth and didn't come from the political production line. If times are good you can see people giving him a whirl. But would you bet your house and your mortgage on him in an economic crisis? No."

That sense that Boris is fun but unserious, and potentially even dangerous, is the greatest impediment to further advancement. The thought of him being handed the nuclear codes is not a prospect likely to reassure any wavering voters.

And yet, for all the doubts, he has built what it is customary these days to refer to as an extraordinarily potent "brand". When the idea of a run for mayor was first mooted it seemed initially like an in-joke cooked up on the comment desk of the Daily Telegraph. Boris being MP for Henley seemed about right, but could he possibly win over millions in a city as diverse as London? It seemed an unlikely prospect.

Guided by the Australian strategist Lynton Crosby, who made him cut his famously mangled mop of hair and avoid obvious gaffes, he defeated the veteran Ken Livingstone, that odious but cunning embodiment of the London Labour machine.

The first term of his mayoralty was a curious affair. In the 2008 campaign I wrote in the Daily Telegraph in support of Boris, describing him as a cavalier to Livingstone's joyless politically correct roundhead. In office the new mayor was certainly hugely entertaining, but other accomplishments were harder to quantify. There were those grey and blue bikes which appeared on the streets of London, although there was always a suspicion that he had inherited a good idea from Ken and then made it his own. Beyond that, even those who voted for him would struggle if they were put on the spot and asked to list his practical achievements.