Harriet Harman scored off David Cameron by hitting him on a very sensitive spot. She accused him of having bad manners. There could scarcely be a more wounding thing to say.

Cameron has been brought up to know how to behave.  On an occasion when standards of conduct were thought to have slipped at Heatherdown, his preparatory school, the headmaster instructed the boys to walk round the playing field, doffing their caps to the corner flags and saying “Good afternoon, Sir”.

A spirit of noblesse oblige was instilled, which meant one took great care not to snub or show contempt for those in less happy or privileged circumstances than oneself. It was in some ways a very poor preparation for Prime Minister’s Questions, where Cameron believes it is his duty, week after week, to score points off a very large group of people in less happy circumstances than himself, namely the Labour Party.

So when Harman asked an admittedly rather tiresome question about why the Prime Minister does not want 16 and 17-year-olds to vote in the EU referendum, Cameron came back with a triumphant passage about how wonderful it had been to see Labour MPs voting last night in favour of the referendum: “the biggest mass conversion since that Chinese general baptised his troops with a hosepipe”.

It was a good line, but Cameron delivered it without any lightness of touch. Instead of treating Labour as an amusing sideshow, he revelled in his opponents’ humiliation.

Which gave Harman the opening she wanted. She began with a concession: “He won the election, he’s the Prime Minister.” That was the cue for happy Tory cheers, of which we heard many during this PMQs, each time a newly elected Conservative rose to speak.

But Harman contended that Cameron’s success means “he doesn’t need to do ranting and sneering and gloating”, and went on to tell him that “frankly he should show a bit more class”.

This column has long been making the same point about Cameron’s performances at PMQs. He could afford to make fewer jibes, and to do so would also be better politics. As Tony Blair showed in the days of his prosperity, the moral high ground is worth seizing and then defending (albeit in Blair’s case the long-term cost of doing this was the development of an intolerable self-righteousness).

Cameron was plainly stung by Harman’s attack. One could tell as much because for the rest of PMQs, the Prime Minister kept saying things like “I hope it won’t be seen as gloating”, before making some disobliging observation about Labour.

There is no need to revel in Labour’s difficulties.To do so sounds hubristic. If Cameron wants to get his own back on Harman, he should start to treat her with elaborate courtesy, and miss no chance to pay tribute to her outstanding abilities. For it is evident, from this performance, that she would make a better Labour leader than any of the candidates who have so far put in for that post.