The conversation that changed the face of the British government, and perhaps the fate of the nation, took place last Sunday in the north London home of Dominic Cummings, the campaign director of Vote Leave, who is now installed as the most powerful official in 10 Downing Street.

When Boris Johnson texted Cummings asking to see him on Sunday morning, the man dismissed by David Cameron as a “career psychopath” was just back from a holiday in Greece and preparing to go under the knife. When Johnson arrived that evening, Cummings explained that he was due to have an operation under general anaesthetic on Wednesday — the day his guest was due to become prime minister — and would be out of action for a