“We regret that, owing to a previous engagement, we are unable to attend.” The trustees of Chequers were treated to the standard brush off by four of the six surviving men and women who have had the run of the Prime Minister’s official country residence. John Major, Tony Blair, Gordon Brown and David Cameron all have something else to do, leaving just Theresa May and the current tenant, Boris Johnson, to grace the gala dinner this weekend to celebrate the centenary of Prime Ministerial retreat in Buckinghamshire.