Donald Macintyre's Sketch: Boris Johnson more interested in tunnels and flyovers than being a party leader? If you believe that…

His return to Westminster, Boris Johnson suggested to incredulous MPs on Monday, was “a vanishingly improbable event”. He was a “proud municipal politician”. After all, who would want to be Prime Minister when he can preside over the “tunnelising” of Hammersmith flyover?

How could the trivial honour of running the country compare for a nano-second with constructing – or at least taking the credit for constructing – a new town centre in West London?

Becoming ever more lyrical on his monthly phone-in, Boris had said that even “hardened” Transport for London engineers had overcome their scepticism about this amazing project. Chaotically gesticulating (you can see it on the LBC webcam) in a generally downward direction, he exclaimed euphorically: “They’re going to take the flyover and make it a flyunder!”

As an answer to Doris of Surbiton about the highly publicised outbreak of hostilities between the fans of Boris and those of George (Osborne) over the future Conservative leadership, this was a classically Johnsonian diversion, emphasising how much “joy” he derived from being Mayor.

As an outright denial that he would contemplate becoming an MP in 2015 in time to take the helm of a defeated Tory party, it was worthless. “So you’re not going into the Commons prior to 2015 because of the excitement of the Hammersmith flyunder?” asked presenter Nick Ferrari not sounding quite as incredulous as he should have done at this ludicrous proposition. “Correct,” Boris replied. “The sheer excitement of the flyunder is keeping…” (Sentence conveniently trails off in cut and thrust of debate.)

This is brilliant. Were it quoted back at him – say at a Tory constituency selection meeting in some ultra-safe shire seat weeks before the election – Boris could reply, without much fear of contradiction, that it was a joke. For however much they are denied, including by Boris, the reports that Osborne has been pressing him to stand as an MP in 2015 to help with the campaign brutally expose his dilemma.

On the one hand he could simply be helping Cameron to victory – or be made a fall guy for his defeat. But if he waits till his mayoral term ends in 2016 he could be too late to have a crack at the leadership. This way, as usual, he keeps his options open until the 11th hour.

Appearing before the Communities and Local Government Committee later in the afternoon, he showed another dangerous side – making a credible case for why the big cities, including London, should keep a bit more of the money they raise in local taxes, while being allowed to reform council tax bands.

Which would increase, for example, what “the Russian oligarch in his stuccoed schloss in Kensington” pays to levels nearer to those in New York or Paris. Even here there was a tilt at the Treasury which seemed to be wrestling with a “psychological problem” over the idea, which, he insisted, would increase overall revenue.

In his phone-in he had shown himself as a master of the reinvented “Butlerism” –named after RA Butler, who once agreed with a reporter that Anthony Eden was the “best Prime Minister we have got”. David Cameron had done a “fantastic job”, he told another caller, so it would be “impertinent to comment” on what he would have done differently. His only “public” difference had been over airport capacity.

But hang on, Boris, if Dave has been so fantastic, what could you have in mind that would be impertinent? And what were the private disagreements?

Never mind that having rubbed along for much of the last century with state school-educated PMs – Wilson, Heath, Callaghan, Thatcher, Major – the class struggle is now down to whether we have yet another old Etonian as Tory leader or slum it with a mere old Pauline like Osborne.

There’s a difference between Boris and Butler though. Which is that RAB (twice) lacked the killer instinct needed to become PM. No one is likely to accuse Boris of that.