Any presentation of evidence that the London Mayor has been at best complacent over the phone-hacking affair might begin by arguing that a damning contradiction was exposed at his irritable press conference on Monday: that on the one hand he clearly affirmed that “I am the democratically accountable authority for policing in London,” while on the other he insisted that the refusal of the Met to re-open its investigation following the Guardian’s revelation that there had been thousands more victims than previously reported was not his responsibility – John Yates had told him there was “nothing new” to be found, so it was proper that Yates, not he, take the blame for the failure.

Is Boris Johnson truly accountable for policing in London, a crisp prosecutor might inquire, or does he simply pretend to be when it suits him? That prosecutor might then turn to motive. Did Boris not have a number of bad reasons for notoriously dismissing concerns over hacking as “codswallop” last September?

One such reason could be that Sir Paul Stephenson, the now outgoing commissioner, had been his preferred candidate for the job, which Boris himself had made available in 2008 by effectively forcing the previous commissioner Sir Ian Blair to step down? Boris had an interest in defending the Met’s inaction over hacking, since in doing so he was also defending his enthusiasm for Sir Paul.

A further motive for Boris’s not pressing Yates to dig deeper would be his personal interest in having the whole hacking story die away. At Mayor’s Question Time last week he candidly explained a deep reluctance to assist police with their inquiries into the hacking of his own phone in 2006. “I had no particular desire to get involved in a court case that revolved around some extremely unpleasant interference in my private life,” he said.

A court case involving Boris as a witness could have dragged up embarrassing episodes from his past. That immediate danger has passed, but our shrewd prosecutor would insist that its legacy lingers. Wouldn’t a fear of the Sun and News of the World bathing this or that indiscretion in the limelight be a powerful motivator against any sort of move to get the Met to give News International a hard time?

And that would bring our prosecutor to the strongest motive of all for passively acquiescing in Yates’s “nothing new” approach: a fear of being damaged politically. Boris, even more than most other politicians, has that huge reason for keeping News International sweet. He’s a celebrity, an act, who provides endless “good copy” but knows only too well that “good copy” can turn bad. Londoners and many others do not even now quite get that Boris Johnson is truly ravenous for power.

His own friends and close colleagues tell us so. Just days before the 2008 election his then fellow Telegraph columnist Simon Heffer warned that Boris is “the most ambitious man I’ve ever known.” The case can surely be made that badgering the Met into turning News International upside down is the last thing the “democratically accountable authority for policing in London,” would do.

The defence against it would be vigorous: counsel for Boris would stress that the Mayor has every right, even a duty, to maintain good relations with some of the most influential and popular newspapers in the land; he or she would pointedly observe that the previous Mayor had his dealings with News International too; a distinction would be forcefully made between the Mayor’s responsibility for setting broad policing strategy and his equal responsibility to not interfere in “operational” matters.

But how real is that distinction in practice? How real should it be? Has taking refuge behind it suited a Mayor who chaired the Metropolitan Police Authority, unseated a commissioner and pledged to take “personal responsibility” for crime and policing in the capital just a little too well recently? Let the jury confer.

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Boris is a Mayor who likes to be liked, and it was when his questioners probed for admissions of failure, arrogance or regret that you could see and hear them get under his skin. The press conference just ended at City Hall is one of the bare handful he’s submitted himself to in his place of work, preferring themed location settings where he can more easily joke, duck and dive. It started smoothly enough, but the hard pressure started taking its toll when the London specialists in the room at long last had a chance to pin him down for longer than the length of a cheery soundbite.

There was, then, no sign of the usual cakes and ale. He was on the back foot, and he could be there for quite a while. People wanted to know what Boris had actually been doing all this time. What assurances had he and his deputy Kit Malthouse extracted from John Yates, the officer who’s just resigned over the hacking affair, that he was exploring the hacking allegations seriously as they increased in gravity and number? As a hacking victim himself, what assurances had he obtained from Yates, when chairing the Metropolitan Police Authority, that he had looked very carefully at the number of other people who might have been hacked? Had he been “asleep on this one?”

Boris stuck to his familiar line, that he’d simply taken on trust what he’d been told. He replied that Yates had been “very clear to me” that there had been “nothing new” coming up. Malthouse backed him up. Clearly, if someone else had made mistakes it wasn’t their fault. But then Boris was asked if he, like Sir Paul Stephenson, should take responsibility for his failures and resign. Did he regret calling phone hacking allegations “codswallop”? Wasn’t his judgement in question here? Did he regret recently praising Rupert Murdoch for his “very considerable contribution” to journalism? Would he apologise for having mocked the people who, unlike him, had stuck at the hacking story? How about for not bothering to make sure Yates took a good, long look at those 11,000 pages of evidence on his watch?

The gathering insinuation was that Boris had just been playing at being responsible for policing; playing, maybe, at being Mayor. It’s the charge his political enemies long to stick to him. The more he offered the excuse that it wasn’t his fault, guv, the less his audience was satisfied. You could sense the temperature rise. Then Jon Snow from Channel 4 News took him through the long history of Met-News International links and personnel crossovers, going right back to John Stevens’s time. Surely it had been clear for years that something wasn’t right?” And that’s why we’re having this investigation!” was the Mayor’s angry bark of a reply. Patience snapped. Bonhomie gone. Boris, behind the mask.