Simon Winchcombe Henry Howard Hartley saw the Prime Minister for the first time that morning, and his father for the last time that night. It happened thus: For the past 200 years, the Hartley family had either taken holy orders, ending their days as bishops, or entered the House of Commons, before joining the Cabinet as a minister of the Crown.

Simon’s father, the Rt Hon. John Hartley PC KBE MC, was no exception and ended a distinguished career as Home Secretary before being elevated to…