Foulds states, [1]
From the evidence of the cartulary of St. Mary's, York (Benedictine), Walter 1st baron married a certain Matilda. In the confirmation by Henry II of grants made to St. Mary's in the time of Kings William I and William II Walter 1st baron had granted to the abbey the tithes of Cotham, Knapthorpe, Hickling, Thurgarton (Notts.), Blankney, Corby Glen, Cotes Cartune and Potter Hanworth (Lincs); in Belton (Lincs.) the abbey received the church, three carucates, half the land and three mills. Matilda, his wife granted the land called Northunda next to Burton Coggles (Lincs.), one carucate in Corby with the wood belonging to it, the tithes of Gamesthorp and Hervingthorpe, the tithes of her desmesne in Lynn and Tudesham and elsewhere. ... Who Matilda was and her parentage is very unclear. An inscription found near the west door of Lincoln cathedral in 1670 refers to William Deyncourt son of Walter 1st baron as of royal descent. Since Walter is referred to on the same inscription as a kinsman of Remigius, William's royal ancestry possibly came through his mother. Matilda was possibly a daughter of King William I. Her chamberlain held land in Hampshire Hatch Warren (Basingstoke Hundred); Goisfridus camerarius filiae regis which he held de rege W. pro servitio quod fecit Matildae ejus filiae. ... She appears in an encyclic letter of the nuns of Holy Trinity, Caen, along with her mother, Matilda, and her sisters Adelaide and Constance. It is possible that Walter married an unknown Anglo-Saxon princess ... but the inscription would be interpreted as casting some doubt on this possibility as it goes on to state that William Deyncourt was educated and raised in the court of King William II Rufus, an arrangement that might, perhaps, suit a closer blood-relative than the son of an Anglo-Saxon princess.
[1] Trevor Foulds, The Thurgarton Cartulary (Stamford: Paul Watkins, 1994), lv-lvi.