Few politicians have been so consistently and so pungently reviled as Castlereagh. Gutter journalists threw mud at his public and private life, commenting openly on his inability to sire children in spite of being married to a ‘fine, comely, good-humoured, playful (not to say) romping piece of flesh’. On a more elevated level, he famously became the butt of vicious attacks by Shelley, Byron and Tom Moore. When he appeared in a theatre or at a public meeting, there was often the danger of being affronted. Many contemporaries agreed with Byron that he was ‘the most despotic in intention and weakest in intellect that ever tyrannized over a country’.

Part of the explanation for this unhappy situation lay in Castlereagh’s capacity to change his opinion of things. Moving from one point of view to another, let alone moving from one party to another, is not, oddly enough, hailed as demonstrating an open and flexible mind, but rather