William was the son of Sibyl Corbet, but it is not known if his father was Herbert FitzHerbert or Henry I of England. Cawley lists him under both, but prefers the former. He also mentions an original document that mentions "Willelmi filii Herberti", suggesting that he was the son of Herbert. The fact that he lived to 1187 also suggests that he might not have been born close to the year that Sybil's illegitimate son by Henry was born.