John McDonnell, the shadow chancellor, has just reaffirmed the commitments in the party’s manifesto to bring rail, water and at least some of the energy sector into public ownership. He also has proposed to look at the range of public finance initiative deals and bring at least some of those back in house.

This raises a lot of questions.

The first is how much it will all cost. That is actually by far the least interesting issue, though that hasn’t prevented it being the focus of most of the media speculation. From the point of view of the headline public finances, borrowing would be increased by the amount spent on nationalisation in the year in which the nationalisation takes place. Government debt would rise by