Why is it so difficult to replace Keir Starmer?

Despite a disastrous set of local and national election results, the prime minister has vowed to carry on regardless. But dozens of Sir Keir Starmer's Labour MPs have now called on him to resign – or at least set out a timetable for when he will leave Downing Street.Surely choosing a replacement – or even triggering a leadership contest – should be straightforward? However, even though there are any number of potential challengers, the PM could well be safer than you think.Niall is joined by Sky’s political correspondent, Lucy McDaid, to discuss the Labour leadership election process, how it differs from the Tories and what needs to happen for Starmer to be ousted.Have you got a question for Niall? Email the show: why@sky.uk