The following post by Rosie Bevan indicates where the identity of Margaret came from:
From: Rosie Bevan (rbevan@@paradise.net.nz)
Subject: The Grey sisters of Heton
Newsgroups: soc.genealogy.medieval
Date: 2002-04-29 00:49:29 PST
Dear List Members
The entry for William, Lord Heron of Ford, Northumberland d.1379 in Complete Peerage, VI p.486, note (i), states that his wife's name was Isabel, parentage unknown, "but it appears from De Banco R., 411 m.218, that she was aunt and coheir of William de Grey, dead by 1362. Margaret, wife of Sir John de Eure, was another aunt and coheir and other coheirs were Sir John Salvayn and Sir William de Felton. The claimant was Sir Thomas de Grey in respect of the manor of Hundburton, (a hamlet now called Humberton), near Stamford Bridge, Yorks."
The heirs of William de Grey were his four aunts, in themselves or their issue. From a petition made to the escheator in 1325, evidence appears that their father was Thomas de Grey. His request was for the wardship and marriage of the heir of John de Eure, as the king had promised to help Thomas marry off one of his daughters. At the same time he also made a petition for the lease of the manor of Duffield at a nominal rent until the majority of the heir, who had married his daughter. . . . [part of post omitted]
The identity of the Thomas de Grey in the petition, is Sir Thomas de Grey of Heton who died in 1343/4. Between 1343 and 1354 Thomas de Grey II of Heton, and his brother in law, John de Eure, were given joint custody of Darras Hall in Callerton Darras, as the heir was a minor aged 9 years old [C.M.Fraser ed., 'Ancient petitions relating to Northumberland'. Surtees Society Publication no. 176. 1966. p.]. In 1311 the escheator north of the Trent had confiscated the lands of John de Grey, son and heir of Juliane de Grey (widow of Sir Robert Grey), in Heton and Norham "par la reson des ordenances feist reprendre en sa meyn le manoir de Heton et un toft et treis acres de terre en Norham el Contee de Northumbr, qe furent a Juliane de Grey et qe par lenemyte et la rebellete Johan fiuz et heir la dite Juliane sicome eschete a la meyn le Roy devyndrent..."[C.M.Fraser ed., 'Ancient petitions relating to Northumberland'. Surtees Society Publication no. 176. 1966. p.23]. It may be significant that the wording implies that the Heton inheritance had come via Juliane de Grey as John was described as her son and heir, not her husband's. He is not even mentioned. The lands were restored in 1312 when Bishop Kellawe formally put Sir Thomas Grey and his wife Agnes in possession of the manor of Heton on 28 October 1312. [Reg. Pal.Dun I 77-78 II 1170-1]. Sir Thomas was most likely a younger son of Sir Robert Grey and Juliane. Like his antecedents, Thomas de Grey was Constable of Norham Castle, Northumberland, one of the two castles of the Palatinate of Durham, on the Scottish border, and he defended it from the Scots in 1318, 1319, 1322 and 1327.
(PRO C 148/128 Subject: Memorandum that Sir Thomas de Grey, Constable of Norham Castle, has undertaken to find men for defence of the castle besides those provided by the Bishop of Durham. (Cal. of Documents relating to Scotland, III, 772) County Northumberland 16 Edw II ) . . .