The future of Labour’s leadership has been plunged into deeper doubt following Wes Streeting’s resignation as health secretary and confirmation that Greater Manchester Mayor Andy Burnham is eyeing a return to Westminster . On Thursday, Makerfield MP Josh Simons said he is willing to give up his seat to allow Mr Burnham the chance to win a by-election, return to Parliament, and challenge Prime Minister Sir Keir Starmer . He said that the Labour Party “owes it to people to come back together” following last week’s defeat in the local elections. It came just hours after Mr Streeting sent a blistering resignation letter to Sir Keir, saying: “‘Where we need vision, we have a vacuum. Where we need direction, we have drift.” The developments intensify days of mounting questions over the prime minister’s position and the prospect of a leadership challenge following dismal local election results last week . But how could a leadership contest be triggered, what would it look like and who could stand?…