Prime Minister's Questions: the old politics is back, and Cameron is in trouble

Welcome back, old politics. Jeremy Corbyn wore a poppy. His shadow chancellor, John McDonnell, finally showed up to support him. Labour MPs shouted and hooted, and Conservative MPs cheered and jeered. And the leader of the opposition decided to ask one simple unanswerable question and stick to it for all his six turns.

"Can he guarantee that nobody will be worse off next year as a result of cuts to tax credits?"

It was uncomfortable for David Cameron. Obviously, nearly everyone on in-work tax credits was going to lose out under Plan A, the one that was thrown out by the House of Lords on Monday. And equally obviously, although George Osborne's clever people at the Treasury haven't worked out what Plan B is yet, many people are still bound to lose out. So he could say, Of course I can't: some people will lose out but it is better for the country in the long run. Or he could say, I'm not going to answer, you'll have to wait for the Autumn Statement.

He chose Charybdis rather than Scylla, and now faces the whirlpool of headlines saying he refused to answer a straight question six times. Worse than that, he came up with a cheap sound bite that equally obviously avoided the problem. He said the House of Lords majority and Commons opposition were "an alliance of the unelected and the unelectable".

The only vestiges of the new politics were Corbyn's light brown jacket (there was an urgent debate in the press gallery: it's not beige – that was Angela Eagle's, next to him – and not yellow enough to be ochre; was it taupe, tan, camel or sand?) and one email question, from Karen, who by chance wanted to ask the same question that Corbyn had just asked five times.

It was a feeble performance by the Prime Minister. In answer to Karen's question, he was reduced to saying that someone in full-time work would benefit from the "tax cut" of the higher personal allowance. This is worth £80 a year against an average loss, even assuming a knock-on benefit from the National Living Wage, of £975 a year for in-work households on tax credits. Corbyn's main achievement was not to interrupt his opponent while he was making a mistake.

On previous occasions, Corbyn has changed the subject, especially for his last question, which deprived Cameron of the chance of a prepared line as the last word. This time he stuck to the same subject and it turned out that Cameron had nothing to say.

The rest of the session was unremarkable, allowing MPs to muse on the question: How on earth did Cameron and Osborne think such huge cuts to the income of the working poor was ever going to work? For once, a Scottish National Party MP asked a good question. Kirsty Blackman, Aberdeen North, asked: "Did the Prime Minister refuse to put [tax credit cuts] in his manifesto because he wouldn't be elected?"

And, in answer to Mark Durkan of the Northern Irish SDLP, Cameron showed what a fine parliamentarian he can be. It was a question about a 23-year-old constituent of Durkan's who had died of cervical cancer, having been refused a scan because of guidelines restricting them to 25-year-olds and older. The Prime Minister gave a sympathetic, persuasive and short answer explaining that the guidelines were not for funding reasons but to balance the side-effects and false positives of the procedure, and promising to investigate further.

But, on tax credits, Cameron looked like a man trying to look dignified on a bicycle going backwards. It is hard to see how he can keep this up until the Autumn Statement on 25 November.