Many years ago there was an observable phenomenon known as the cost of the Today programme. Diligent think tankers used to add up the promises made by ministers in the course of interviews on Radio 4’s programme and estimate the impact on the public finances.

That has been replaced by the curse of the Sunday morning political television show. Eighteen years ago, in January 2000, Tony Blair took to the sofa to be interviewed by the late Sir David Frost and promised to raise health service spending significantly, to match the European Union average. Whether that pledge sank in among every viewer is unclear, but it did have an impact on one: an outraged Gordon Brown, who had just witnessed the prime minister revealing what