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William Crassus, or Le Gros (d.1219), was the son of Stephen Le Gros. William was from an Anglo-Norman baronial family long established in central Normandy. He inherited Sodbury from his uncle, William le Gros, 1st Earl of Albemarle, primo-genitus. He was granted a licence to hold fairs and markets in (what is now) Chipping Sodbury in 1217.
William was appointed Seneschal of Normandy in 1203 by King John of England. He was a noted kinsman and follower of the elder William Marshal, 1st Earl of Pembroke.
He married Margaret, daughter of Robert Fitz Warren. By Margaret he had at least one son, his namesake, William Crassus, or Le Gros.
- Étienne le Gros