Donald Macintyre's Sketch: Nick Clegg plays PM at PMQs and Harriet Harman tells jokes

Late in PM’s Questions the (very) right-wing Tory backbencher Peter Bone rose to deliver his magisterial verdict on Nick Clegg’s substitution for the absent David Cameron. “Hasn’t the acting Prime Minister been outstanding today?” he asked, adding that “anyone listening on the radio” would have thought it was Cameron himself. (This wasn’t a huge compliment since Bone is hardly the PM’s greatest fan). But then the only half-sarcastic pay-off: “I think that the Right Hon Gentleman is turning into a Tory.”

Possibly for the first time in his career, Bone had a point. Were the Liberal Democrat leader to seek a coalition with Labour in 2015, word-eating would have to happen on such a gargantuan scale that you would fear for the safety of the Clegg epiglottis. His description of the energy price freeze Ed Miliband would presumably impose as “economically illiterate” and “total, utter fantasy” was just one example.

These Labour-baiting lines were delivered with Cameronian savagery but without the jokes. Harriet Harman did the jokes, with mixed results. Faced with the routine onslaught from Clegg on Labour’s dependence on its “union paymasters”, she suggested he “leave it to us to worry about our party members, especially as so many of them used to be his”. Which wasn’t bad. But then she yelled above the uproar: “He says he’s a brake on the Tories …but … even I know the difference between the brake and the accelerator!”

This reference by Ms Harman to her conviction for driving without due care could be seen as self-deprecating. The problem with self-deprecation is that it rarely works in this zoo-like atmosphere. It’s a bit like the child who impulsively admits in the playground that he’s rubbish at football and finds that his nickname is “rubbish” for ever afterwards.

If anything, Ms Harman was better on the specifics, notably the bedroom tax. Castigating Clegg for agreeing to it, she described him as “the very best deputy a Conservative Prime Minister could ever wish for.”

Come to think of it, she might have to eat some words too, if a Lib-Lab coalition were in sight. Or maybe she’s still holding out for a Vince Cable leadership coup.