The failure of many of the UK’s recent prime ministers, who have passed through Downing Street in quick succession, seems easy to explain. Theresa May couldn’t do what she promised and didn’t “get Brexit done”. Boris Johnson broke his own rules, and the law. Liz Truss failed through sheer incompetence. But Keir Starmer won an election by a landslide and led his party to victory after 14 years out of power. So why is he looking at a probable leadership challenge after less than two years in offi