‘The best prime minister we never had’ is not an epithet exclusive to Rab Butler. Widely applied to the late Denis Healey, it was also said of Hugh Gaitskell, Iain Macleod and Roy Jenkins. (More recent candidates would include Michael Heseltine and Kenneth Clarke.) All had arguably greater intellects than the prime ministers they ended up serving, all enjoyed significant popularity in the country and all were committed to the centre-ground of British politics.