Today, Doncaster. Yesterday, Teesside, West Yorkshire and Derbyshire. All lifelong Labour heartlands. So why has Theresa May been campaigning there? The answer – Conservative strategists believe they can persuade Labour supporters who switched to Ukip at the last general election to back Theresa May this time,turning those seats into a sea of blue. But with talk of a Corbyn surge, is that strategy still on track?