THURSTAN DE MONTFORT (b), brother and heir. His name appears in the account of the sheriff of Berks in 1130, He had succeeded his brother by the spring of 1141, when the Empress Maud at Winchester gave him a charter for a market every Sunday at his castle of Beaudesert, near Henley in Arden. He attested several charters of Roger, and one of William, Earls of Warwick, to the College of Warwick, his name coming usually after those of the Earl's immediate family. He also attested, on the Earl's side, the agreement made at Devizes in 1153 between Henry, Duke of Normandy (afterwards Henry II), and Ranulph, Earl of Chester. In 1156 he owed the King 20 marks for his land in Rutland. In 1166 he held under three tenants in chief of the Earl of Warwick 10 1/4 fees (Beaudesert, &c.), of Roger de Mowbray 3 3/4 in Yorks, and of Robert de Stafford 1/4 (Henley in Arden). After his brother's death he confirmed the grant of Wing to Thorney Abbey for the souls of himself, his wife and children, and especially of his brother Robert. He made a gift to Guisborough for the soul of his brother Henry (1155-70). His name disappears from the Pipe Roll after 1170. He married Juliane, daughter and coheir of Geoffrey MURDAC. [Complete Peerage IX:120-1, (transcribed by Dave Utzinger)]
(b) [actually about Thurstan's elder brother Robert, but it also applies to Thurstan] The ancestry of this family cannot be given any certainty. The only place in Normandy named Montfort is Montfort-sur-Risle. Miles Crispin, in his Life of William, the third abbot of Le Bec, thus describes the abbot's origin: "Willelmus, nobli Northmanorum prosapia originem trabens, in veteri castro super Rislam quid dicitur Montfortis, claris parentibus est exortus. Pater ejus Turistinus, mater Albereda dicebatur, Rogerii de Bellomonte patris Roberti comitis ex sorore neptis." At the time of the abbot's birth (circa 1054) the seigneur of Montfort was Hugh, but the language used suggests that Thurstan was a member of that family, the more so as he bore the same name as its founder, Thurstan de Bastenbourg. Roger de Beaumont was father of Henry, 1st Earl of Warwick, who would thus be Thurstan's first cousin by marriage; Montfort-sur-Risle is about 15 miles distant from Beaumont-le-Roger. The repeated occurrence of the name Thurstan among the Montforts of Beaudesert, undertenants of the Earls of Warwick, taken in conjunction with the above facts, points to somewhat strongly to the probability of their being a younger branch of the family of Montfort-sur-Risle.
(c) [actually about Thurstan's elder brother Robert, but it also applies to Thurstan] Robert was probably son of Thurstan de Mundford, who, as one of the barons of Henry, Earl of Warwick, attested the Earl's charter to Abingdon in the days of Abbot Reynold, who d. 1097. This Thurstan was very possibly of the house of Montfort of Montfort-sur-Risle (see note "b" above), but the connection has not been proved. Robert's brother Thurstan held land in Berks in 1130, and property there was held by his descendants till the death of the last heir male. That there was a Thurstan preceding Robert at Preston, and ancestor of Piers, who d. 1287, was proved to the satisfaction of the jury of the Hundred in 1275, who found that Preston used to be the demesne of William the Conqueror, who gave it to the Earl who was at Warwick, whose name they did not know, to hold by the service of 1 1/2 knight's fee, and the said Earl gave it to one Thurstan, ancestor of Piers de Montfort who was killed at Evesham, who held, and his widow held, by the same service of the Earl of Warwick.
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Note: The Plantagenet Ancestry, by Turton, p. 88, has Thurstan as son of Adeline de Beaumont & Hugh de Montfort, son of Gilbert de Gant & Alice (Jeanne) de Montfort, daughter of Hugh de Montfort mentioned by CP above. There is partial support for this by Ancestral Roots (the Gilbert de Gant & Alice de Montfort marriage), as well as full support from Burke's Extinct Peerage. I find nothing in the above from CP persuasive enough to say the other sources are wrong.
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Thurstan de Montfort, being enfeoffed of divers fair lordships by Henry de Newburgh, the 1st Earl of Warwick, erected a stony castle, called Beldesert, at the chief seat of his family in Warwickshire, which it continued for several subsequent ages. To this Thurstan, who d. before 1190, s. his son, Henry de Montfort. [Sir Bernard Burke, Dormant, Abeyant, Forfeited and Extinct Peerages, Burke's Peerage Ltd, London, England, 1883, p. 377, Montfort, Barons Montfort]