Two years ago when the Tories won the Hartlepool by-election at the local elections, the political mood was summed up by a 30ft inflatable blow-up of the then prime minister Boris Johnson looming over the town. He was photographed in front of it as part of his victory lap. The message was clear: under his leadership, the Tories were insurmountable and no seat was safe for Labour in what used to be the party’s heartlands.