Among the lesser consequences of the Pope's current brush with Islamic sensibilities has been to cast a sudden light upon a curiously named but otherwise obscure Byzantine emperor, Manuel II Palaiologos. Besides having a long life - he reached 75 - and a pretty long reign (1391-1425) for a ruler in a permanently precarious position, it turns out that Manuel has local interest: he is the only Byzantine emperor to have visited England.
It happened in the year 1400, when Manuel was the honoured Christmas guest of King Henry IV in, of all places, Eltham, now a district of south-east London and best known as the location of Stephen Lawrence's murder. In those days Eltham Palace was a handsome royal retreat set in good hunting country.
The chronicler Adam of Usk was taken with the exotic emperor and his entourage, who were "most devout in their services, which were joined in as well by soldiers as by priests" - the inclusion of bodyguards in royal acts of worship being rather democratic for the time. Manuel, for his part, wrote of Henry that he was "most illustrious", and praised his intelligence, his might and his understanding. Interestingly, however, he thought Henry's courtiers were overdressed.
Manuel was to be disappointed. He had come all this way, leaving behind a tiny rump of empire under siege from the Ottomans, in the hope of finding a Christian monarch who would help him.
"I'm afraid he was over-optimistic," says Professor Judith Herrin, a Byzantinologist at King's College London. "Henry never sent any real assistance in terms of fighting forces, which was what Manuel needed." Though Shakespeare shows Henry frequently dreaming of leading a crusade against the Turks, it seems that in reality he preferred to fight the French.