The year's first prime minister's questions, so we craned forward so as to miss nothing. And we didn't miss it. David Cameron's bald patch is still there! Yet, and this seems to me almost incredible, it has moved. Last summer it was on the left of his head as you saw it from behind. Now it has shifted to the right, nearly two inches away. How is this possible? Is he getting an Elton John-style weave? Or is it a matter of combing the hair in different directions, so that covering the bald patch in one place makes it re-appear in another? Whatever the truth, it is clear that it matters a great deal to the prime minister. Do you think he has a valet to brush his hair each morning in the most artful manner possible? Or does he have two mirrors, one behind the head, like they do in the barber's?

Politicians would say that this is an entirely trivial matter. They can't be bothered with such silliness when there are important things to be done. In their case, exchanging personal abuse. It was the nastiest PMQs since Ed Miliband became Labour leader.

Actually, Ed started off pretty well. He pointed out, silkily, that Mr Cameron had promised that in the banks owned by the public, nobody would get a bonus of more than £2,000. "Can the prime minister update us on progress in implementing this promise?" Cue loud laughter from Labour, and Cameron insisting that it was the fault of the Labour government – as if they had no idea of what the national accounts looked like until the fateful day they took office and suddenly realised: we were in debt! The PM decided to be rude. Specifically he decided to be rude to Alan Johnson. "I know the shadow chancellor can't do the numbers. There is no point in Wallace asking Gromit about that one!"

This smelled of sweat pouring off spin doctors, aides, special advisers and all the young persons who sit around in fuggy, Diet Coke-filled rooms, providing lines for the party leaders. The pair argued pointlessly about whose bank taxes would raise most money. Ed Miliband decided more personal abuse was needed. He quoted Vince Cable, who had said " 'if you keep people in the dark, you will grow poisonous fungus' – on this occasion he wasn't thinking about the chancellor of the exchequer." Ooh, missus!

The PM came up with another perspiration-stained one-liner. "That was such a long question that he should be thinking about a TV career and get his brother to run the Labour party!" (Late as ever: David Miliband's latest crazed scheme is to half-manage Sunderland football club.)

So Ed described John Redwood as a Vulcan, a reference to Star Trek, a show which was generally set on weird planets which, amazingly had the same atmosphere as Earth and where the inhabitants spoke English, which made them faintly resemble the House of Commons.

So Mr Cameron decided to finish with dignity saying: "We've got a shadow chancellor who can't count and a Labour leader who doesn't count ... he is the nothing man!"

And on that thoughtful note he left the Chamber, no doubt for an urgent appointment at the Michael Fabricant Trichology Clinic (first consultation free).