Boris Johnson is on the top deck of a Routemaster drinking a pint of bitter. It is nearly a year since he banned alcohol on the city's public transport, his first act after being elected Mayor of London on 1 May. But here he is, three-quarters of the way through a pint of frothy best. "You're quite right, I shouldn't be seen with this, should I?" he says in that furtive I've-just-been-rumbled way.
It is sometimes said of Boris that he sees himself as above the rules. On this occasion, though, as he points out, we are not on public transport. It is St George's Day, which Boris is keen to wrest back from the British National Party, and he is on a tour of the City, stopping off at Leadenhall Market to see the traders he visited on last year's campaign trail. Chaos descends as he is ushered from cheese-makers to Morris dancers, pressing the flesh.
"I love my job, it's fantastic!" he enthuses. Being Mayor of London probably wasn't on Boris's "to do" list as he progressed from Eton to Oxford to journalism to politics, and even a year on, City Hall seems an unlikely place to find him. With its garish signs for multi-faith meditation rooms and rows of disabled toilets, not much seems to have changed since the days of Ken Livingstone. Even now Ken sits in on assembly meetings, which Boris likes. "I'm a bit disappointed when he's not there," he says. There is a curious affection between the two men. "I pay tribute to Ken Livingstone because he made this the job it is. It is a massive job. I think it's an important job."
An important job, but while for Ken it was his life, one cannot help suspect that Boris is merely passing through on his way up. Isn't the problem with ambition that once you have landed a prize, your eye moves on to the next big thing? "My appetite for power is glutted," he says, "This is the single most exciting and interesting job – I don't know how much more emphatic I can be about this."
Some say he was a better force for Conservatism as a journalist, although he continues to write a column for The Daily Telegraph.
"Journalism is a brilliant job, but all the time what you're doing really is chucking a rock over the wall and waiting for the tinkle of the greenhouse – you're not taking responsibility, and after a while you start to think maybe it would be a good idea to try and do something purposeful."
But Boris hasn't lost the urge to lob the odd grenade. Last week, he stole the lead on David Cameron in denouncing the 50p tax, and his support for grammar schools and plans for a Thames estuary airport diverge from the Tory line. All innocent enough, he insists, but it seems he just cannot resist winding up Dave. Take his remark in an interview to the Evening Standard last week, when asked if the mayoralty was a dress rehearsal for No 10: "If like Cincinnatus I were to be called from my plough, then obviously it would be wrong of me not to help out."
When I press him on his ambitions he does little to dispel speculation, quoting Michael Heseltine's famous non-denial denial. "I cannot foresee the circumstances in which I would be called upon to serve in that office," he says. "I am a passionate supporter of and believer in David Cameron, George Osborne, Kenneth Clarke and all the rest of them for the next Conservative government. The Standard's suggestion that he will serve for only one term was swiftly rebutted by City Hall. But an article by Boris's biographer, Andrew Gimson, in the current edition of The Spectator, the magazine he edited until 2005, gives a well-reasoned argument why David Cameron, and more specifically, George Osborne, should be worried.
Gimson believes Cameron intends to serve only two terms as prime minister, after which Johnson could have served two terms as mayor and would be ready to return to Westminster. He is already the Conservative with the biggest legislative mandate in the country, and the only Tory known by his first name alone.
Already Boris feels he has made a difference to London. "The thing I'm most proud of at the moment is that we've greatly expanded the number of people in uniform on public transport. According to the latest London survey, that really seems to be having an effect on people's sense of safety. All politicians always try to take credit for everything they possibly can, but I looked at those figures, and there was such a noticeable kink upwards in the last year, I thought maybe, conceivably, it really was something to do with us. If it was that would be great."
His critics say he has not done enough. He admits to being surprised at "how incredibly difficult it is to get things done". The next big thing is, he says, the Olympics. "They will be just the right thing for this city in a downturn. People will come together and just enjoy them as we finally leave the recession behind."
The serious new Boris is impressive. There are fewer hesitations, and although his speech is peppered with classical references, there is just enough committee-speak in there to sound convincing. David Cameron was worried Boris would mess things up as mayor; perhaps it would be better for him if he did.