At Runnymede on 15 June 1215, King John sealed an agreement ‘for the reform of our realm’. At the head of the list of witnessing barons appeared the name of William Marshal, Earl of Pembroke (c 1147–1219). Sixteen months later, on John’s death and amid civil war, a French invasion and a haemorrhage of noble support, the nearly septuagenarian William emerged as regent for the new boy king, nine-year-old Henry III. In private William apparently insisted, with a flourish of almost Churchillian sentimental exaggeration: ‘If everyone abandons the boy but me, do you know what I shall do? I will carry him on my back, and if I can hold him up, I will hop from island to island, from country to country, even if I have to beg for my bread.’ For the remaining three years of his life, William acted on behalf of Henry, a distinct medieval rarity – a non-royal ruling a kingdom. He presided over a government that defeated the rebels, expelled their French allies and restored the credentials of the monarchy. Most notably, in a canny move to reconcile his opponents and those wary of the excesses of previous monarchs, he issued a redrafted version of Magna Carta, elevating it from the failed 1215 wartime compromise into a lasting manifesto for consensual politics, law and administration.

A younger son from a background of aspirational minor nobility, William, through a combination of good looks, charm, international-class athleticism, military prowess, ambition, self-interest, a mastery of court intrigue and longevity, lived life at the centre. He was personally acquainted with every English king who reigned between 1135 and 1272,