The Conservative benches in the House of Commons looked stunned. Up in the Press Gallery, the assumption was that they were still absorbing the blows of the weekend. We couldn’t see what the ministers on the front bench could see.

Penny Mordaunt, who had been joking with the chief whip, was now whispering urgently to Oliver Dowden. She looked along the front bench to the Speaker’s Chair, where Rishi Sunak was still waiting to come in. After what looked like a small internal debate, she hustled over to him with a warning that he was walking into an ambush.

With less than two minutes to go before Prime Minister’s Questions, Natalie Elphicke, the Tory MP for Dover, twice voted Most Bonkers at the annual awards of the Association of Right-Wing Nutters, Loons and Fruit Loops, had taken a seat on the Labour benches, directly behind Keir Starmer.

Sunak, by the time he sat down, had a smile on his face. Perhaps he knew the cameras were on him. Or perhaps he just saw the funny side. Elphicke’s interventions in Parliament are generally along the lines of demands that asylum seekers be machine-gunned in the Channel. Her departure from the Conservatives will have the effect of shifting the party’s centre of gravity slightly to the left. Meanwhile Elphicke’s conversations with her new colleagues will be interesting, in the Chinese-proverb sense of the word.

As Starmer announced her defection, there was a genuine battle for comprehension among the press. Were there two Natalie Elphickes? Could this be real and, if it was, what would be next? Mark Francois defecting to the Greens? Jacob Rees-Mogg joining the SNP?

The Speaker called time and Starmer led his very newest MP away

Conservative MPs seemed to be conducting similar mental calculations. By-elections they can understand. The departure of centrist colleagues they can get their heads around. But had you told any of them on Wednesday morning that Elphicke was going to be spotted in Starmer’s office that afternoon, they would have assumed it was to slip arsenic into the Labour leader’s tea. Down below us Tory chairman Ric Holden, usually ready with an abusive heckle for the Labour leader, just stared.

Meanwhile there are practical considerations. The endless debates about general election timing all assume that the decision is Sunak’s. But with two defections and a by-election, he has lost three MPs to Labour in a fortnight. At this rate of attrition, his majority will be gone by the end of August.

Much of this is beyond his control, but we have to wonder how furious with him Elphicke must have been to defect. She apparently believes Sunak has betrayed the legacy of Boris Johnson. But on Wednesday she accused the prime minister of leading a government that is “a byword for incompetence and division”, which sounds quite faithful to the Boris vision.

Starmer was, unsurprisingly, smug. The prime minister, he said, had lost MPs, by-elections, councillors, “half of his party’s mayors and a leadership election to a lettuce”. How much longer, he went on, “before he takes the hint?”

Sunak tried to rise above it, quoting the sainted Tony Blair: “He can be as cocky as he likes about the local elections; come a general election, policy counts.” Behind Starmer, Labour MPs cheered and made gestures suggesting that they would like nothing so much as to test this thesis. Behind the prime minister there was silence, immobility.

Starmer replied by telling Sunak that, in all “the many places he calls home”, Labour are in local government. I had missed the news about Santa Monica going red.

Only towards the end of the session did we begin to detect stirrings of life on the Conservative side of the chamber. On the far backbenches, just as Sarah Dines asked Sunak if he agreed that what she and her colleagues bring to Westminster is “plain old-fashioned common sense”, Alec Shelbrooke said something, hiding his mouth behind his phone. Next to him, Jeremy Wright’s shoulders began to shake as he tried to maintain a straight face. On the other side, Gary Sambrook was openly chortling. Were they contemplating the idea of Elphicke setting out her views to a meeting of the Parliamentary Labour Party? Who knows, but one should, as the song says, always look on the bright side of life.

The Speaker called time and Starmer led his very newest MP away. They were, as it happens, walking in the direction of the House of Lords, but we probably shouldn’t read too much into that.