An unusual visitor dropped in on National Grid’s substation at Highbury, north London, late last year. Jeremy Corbyn was there to meet John Pettigrew, the company’s boss, and get a glimpse of National Grid’s 32km network of tunnels — developed at a cost of £1bn — that supplies electricity to the capital.

Pettigrew had chosen the location carefully. A short cycle ride from Corbyn’s home, it is surrounded by affordable housing and supplies free heat to a nearby primary school. The meeting had been supposed to last an hour, but Corbyn stayed and chatted for four. The opposition leader met some apprentices, and he and Pettigrew parted on friendly terms.

A visit today would be frostier. The Labour Party set out plans last week to