this retelling of the murder of St Thomas Becket is sublime

In AD 1170, Thomas Becket, a former king’s favourite and a rebellious Archbishop of Canterbury, was assassinated in his cathedral. The fatal blow was so strong that it lopped off the top of his head and shattered the sword in two.

As a fascinating new show at the British Museum demonstrates, killing this critic of temporal power generated a religious phenomenon. People collected his blood: one man rushed some home, soaked up in fabric, to give to his wife. She was instantly cured of her paralysis, the first of hundreds.

Three years after his death, Thomas was made a saint, which is swift even for medieval standards. Reliquaries housing bits and pieces of him cropped up as far away as Sweden: he was an export, a political symbol and a tourist magnet, painting a picture of the 12th century that took me by surprise.

Forget the Monty Python image of toothless peasants gathering mud. Cosmopolitan England was part of an empire that stretched from the Scottish border to the Pyrenees; the London of Thomas’s childhood thronged with trade from China and the Middle East. At Canterbury Cathedral, he would’ve enjoyed a state-of-the-art water-pumping system, illustrated at this exhibition in the pages of a psalter. Far from being a Dark Age of intellectual stagnancy, Norman England bubbled with debate over the right way to govern, and Thomas, we are reminded, was an inspiration for Magna Carta.

The break with Henry II, which began when Thomas refused to serve as his chancellor as well as archbishop, frustrating Henry’s dream of an alliance between church and state, was as philosophical as it was personal. Henry wanted clerics tried in secular courts; Thomas wanted to preserve separate justice. I suspect the average punter today would agree with the king, and perceptions of Thomas capture the ebb and flow of British attitudes towards Catholicism.