His father and his elder brother Henry were killed at the Battle of Evesham in August 1265. The younger Simon had been slow to bring his forces from London, and had seen them and their banners captured by Prince Edward, who then used the banners to trick Simon's father. The younger Simon came to Evesham just in time to see his father's head atop a pike. The younger Simon tried to raise a rebellion in Lincolnshire, but this had petered out by Christmas.