7 hours ago Chris Mason, Political editor and Iain Watson, Political correspondent PA Media Prime Minister Sir Keir Starmer is facing the first explicit threat of a leadership challenge from one of his MPs - but just how much of a threat does it pose to him? The former minister Catherine West's interview with Radio 4's PM programme immediately raised eyebrows around Westminster. Plenty of Labour MPs from across the party have told us they didn't see it coming. "What the hell is going on?" said one figure on the Left. "Crikey," said another. Equally succinctly, a former frontbencher declared West's intervention to be "bonkers". But another told us they were aware of her plans from messages that had been shared between some Labour MPs. They characterised her intervention as an expression of exasperation and frustration. "It is a howl of pain," said one minister, reflecting that so many people in the Labour Party this weekend are hurting. "Many have lost their social circle in their patch.…