Following the leak of video footage showing Downing Street advisers joking about last year’s illicit Christmas staff party, the Westminster village turned up at Prime Minister’s Questions today hoping to bury Caesar, not to praise him.

Judged by that yardstick, for Boris Johnson just to have got to the end of a torrid session without having suffered a knockout blow must rank as some kind of achievement.

But he only reached the final bell by leaving many of his key officials fearing for their careers after he announced an inquiry by the Cabinet Secretary and pledged that if rules were found to have been broken “there will be disciplinary action”.

Keir Starmer will have left content that his own strategic objectives had largely been fulfilled

The enterprising Daily Mirror political editor Pippa Crerar instantly understood that there is nothing like fearing you are about to be made a scapegoat to persuade people to get their retaliation in early and tweeted: “If you’re one of those Downing Street staff who Boris Johnson has just thrown under a bus, my DMs are open and my sources are always protected.”

Johnson’s short-lived press secretary Allegra Stratton may well feel there will be a runaway bus — presumably an environmentally-friendly, all-electric one — earmarked just for her after the PM said he had been “furious” to see the clip of her making light of social distancing requirements and apologised unreservedly for the impression created.

Labour leader Keir Starmer will have left the session content that his own strategic objectives had largely been fulfilled: he got Johnson to agree to make evidence about what went on available to the Metropolitan Police should they request it and he underlined the gravity of Downing Street officials ignoring rules they had imposed on everyone else.

He achieved this final goal by quite skilfully contrasting the high jinks in No.10 during last winter’s Covid wave with the grim reality of what Johnson’s lockdown rules meant for everyone else. There was the heart-breaking story of one woman, Trisha, who had not been able to visit her dying mother, preceded by a serviceable joke that “Ant and Dec are ahead of the Prime Minister on this” in reference to the diminutive Geordie duo having referred to the furore on their “I’m A Celebrity…” prime-time TV show.

Starmer’s reminder that the Queen had been forced to sit alone at the funeral of her husband of 73 years and withering comment that “sacrifice gives the moral authority to lead” was probably the moment that will have caused most discomfort on the Tory benches.

Johnson meanwhile had switched from being categorical that no party had taken place to a new formula: “I have been repeatedly assured that there was no party and no Covid rules were broken.” That amounted to yet another sign that he was preparing to lay down his friends for his life.

When one of his own backbenchers warned him not to implement new Covid restrictions against Omicron as a “diversionary tactic” there was a sharp intake of breath across the House. That landslide election victory suddenly seemed a very long time ago.

Troubles mounted further for the PM when an opposition MP asked him whether there had been a party in Downing Street on 13 November last year, a claim that seems to have emanated from his vengeful former adviser Dominic Cummings. This gathering, unlike the staff knees-up, is actually alleged to have taken place in the PM’s flat.

If Starmer fell down on one point it is that he did not use his last intervention to throw out a “zinger” soundbite, instead finishing on a process point about cooperating with the police. But a few minutes later Labour’s Rosena Allin-Khan, a hospital doctor, provided just such a moment when she regaled him about her own experiences working on the Covid frontline before asking the PM: “How can he sleep at night?”

Johnson has spent the past few months alienating many of those who were once part of his political base

The usual Boris Johnson tricks — accusations that Labour were “playing politics with Covid”, complaints that he was being asked about events from a year ago rather than what the Government is doing right now, making superfluous references to other, safer issues such as drugs policy and the latest tough-sounding immigration legislation — did not work well today.

This is not, as some have suggested, another “Barnard Castle” episode. It is much worse than that. Back when Cummings was being defended against charges of breaking the first lockdown, Johnson still had the enthusiastic support of almost everyone who had voted Tory in December 2019 and his adviser was widely seen to have acted out of a desire to protect his family.

Johnson has spent the past few months alienating many of those who were once part of his political base, whether by committing to radical and prohibitively expensive policies to combat climate change, failing to prioritise action on the Channel migrant crisis or whacking up taxes and being feeble on culture war issues. A new round of “Plan B” Covid restrictions will further enrage a substantial part of the Tory tribe.

The list of those ready to circle the wagons around him has become considerably shorter, while the list of his enemies and detractors gets longer by the day. In Starmer he faces an opponent who lacks a knockout punch, but may yet prove adept at gradually tightening a noose around his neck.

Should the voters of North Shropshire decline next week to send a new Tory to the Commons in place of Owen Paterson then a great unravelling of Johnson will be well under way.