Why does Balls, described less than two months ago by the PM as the "most annoying person in modern politics", so wind up Cameron? It's worth paying attention to the nature of the shadow chancellor's serial provocations, for Balls is a master sledger, a match for Australia's cricketers in their heyday. He has studied the acoustics of prime minister's questions, realising that there is a moment, just as a roar or cheer subsides, when, if he speaks directly to Cameron at normal, conversational volume, the PM will hear him even if the microphones do not. His taunts follow a consistent theme: that Cameron is not up to the job. "You're supposed to be the prime minister," Balls will say – or, with equal, quiet menace, "You don't know the detail, do you?"
The former Aussie captain Steve Waugh described the sledger's aim as the "mental disintegration" of his opponent. Cameron's puce-faced outburst will encourage Balls in the belief that he is making steady progress towards that goal. Privately, the shadow chancellor wonders if he has struck a nerve, whether, for all Cameron's Bullingdon bluster and Etonian confidence, the PM harbours doubts about whether he has sufficient intellectual heft for his post – in which case, Balls has found him out.