I finally found a French account of William Longespee’s capture at the Battle of Bouvines.
Indeed, the Bishop of Beauvais, having seen the brother of the King of the English, a man of incredible strength whom the English had on this account nicknamed “Longsword,” overthrow the men of Dreux and do great harm to his brother’s battalion, the bishop became unhappy, and since by chance he happened to have a mace in his hand, hiding his identity of bishop, he hits the Englishman on the top of the head, shatters his helmet, and throws him to the ground forcing him to leave on it the imprint of his whole body. And, since the author of such a noble deed could not remain unnoticed, and since a bishop should not be known to have carried arms, he tries to hide as much as possible and gives orders to John, whom Nesle obeys by the right of his ancestors, to put the warrior in chains and to receive the prize for the deed. Then the bishop, throwing down several more men with his mace, again renounces his titles of honor and his victories in favor of other knights so as not to be accused of having done work unlawful for a priest, as a priest is never allowed to be present at such encounters since he must not desecrate either his hands or his eyes with blood. It is not forbidden, however, to defend oneself and one’s people provided that this defense does not exceed legitimate limits.
-The Philippiad of William of Breton, Song XI, verses 538-58
Actually I’m rather amused now because it loosely parallels what happens to a character in one of my more recent books, though that wasn’t my intention.