A little under a year ago, it emerged that builders working on the redevelopment of the National Gallery’s Sainsbury Wing had discovered an unusual time capsule embedded in a pillar they had been instructed to knock down. It contained a letter signed by Sir John Sainsbury, who, along with his brothers, had thrown the museum a £50 million lifeline to realise the extension in 1990; and clearly, he wasn’t happy with the way his money was being spent.