Keen local historian Jon Gunson has compiled this series entitled Friday Knight - knights who fought at the Battle of Lewes - giving each a brief biography.
Sir William was Lord of Berling and Seale, in Kent, and Constable of Rochester Castle from 19th May, 1260. (By 1264, Roger de Leybourne had taken over).
He fought for the King at Lewes, and escaped after the battle. According to Horsfield and Mantel, “he held fourteen knight’s fees in Hammes, of the honour of Warren.” From 1222 or so, that village just north of Lewes was named after his family - thus Hamm Say, or Hamsey.