In recent years, many but not all politicians have approached the issue of Brexit like you or I might approach an electric fence. Avoid getting too close and, if needs must, climb over it gingerly.

Burnham has no such reservations: he has repeatedly said in the last few weeks, let's talk about Brexit. He reckons the current deal between the UK and European Union isn't good enough.

Over the weekend he went further, suggesting he wanted an "honest discussion" about freedom of movement. Countries in the EU's single market have to sign up to freedom of movement, which lets people live and work in all the countries that are part of it.…