Like many of Europe’s long-established parties of the left, the UK Labour Party is in big trouble. In last December’s election, Labour wasn’t just beaten, it was humiliated, losing its grip on working-class heartlands in the midlands and the North. Stephen Sackur interviews Len McCluskey, who will have a big say in the choice of the party’s next leader. He leads the Unite Union, which is Labour’s biggest financial backer. Who can save Labour from a slow death?