[1] He was the grandson of Sir Robert de Ros, one of the twenty-five barons who guaranteed the observance of Magna Carta, and Isabel of Scotland, an illegitimate daughter of William the Lion, King of the Scots, by Isabel, a daughter of Robert Avenel.
249-50); but these writs, issued by Simon in the King's name, are no longer regarded as valid for the creation of peerages.
[5][4] On 3 July 1257, Ros obtained from Henry III a grant of the free warren, in the lordship of Belvoir, by which the boundary was determined.
In 1261 he obtained from the king the grant of a weekly market, to be held at Belvoir, on Tuesday; and of an annual fair on the feast of St John the Baptist, to continue for three days.
In 1264, he was one of the insurgent barons who defeated Henry III at the battle of Lewes, and took him and the prince prisoner, confining them in Farleigh Hungerford Castle.