 Keir Starmer is a former head of Britain's prosecution service and has loyally approached arguments, often forensic, but also occasionally a little low energy. This event was the last moment when he might say something to damage his commanding lead in national polls that has stuck throughout this weeks-long election race at around 20 points. But he stuck instead to some familiar talking points about the chaos of the Conservative Party's 14 years in power and the need for a change in the country's politics.