Have you heard the old joke about how to tell when a politician is lying? Answer: their lips move. With Jeremy Corbyn, that rule doesn’t apply. The Labour leader struggles to say things that he does not sincerely believe, even when he has been persuaded of the need to say them.

As far as Corbyn is concerned, the gag needs to be modified slightly. How can you tell when the leader of the opposition is lying to you? Answer: his words are in print. Corbyn’s favoured medium when he has to say something that makes him uncomfortable is the written word.

Perhaps “lying” is a bit strong, but Labour has two leaders: the Corbyn who says the things that he knows he has to, who