Turton's Plantagenet Ancestry has Theobald as brother of Geoffrey & Philip de Valognes, son of Robert de Valognes & Agnes, however Raymond Phair, in a posting to soc.genealogy.medieval, has a quite different ancestry.
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Following excerpted from a posting by Raymond Phair to soc.genealogy.medieval:
From: Raymond Phair (rphair@@CapAccess.org)
Subject: Ranulf de Glanville
Newsgroups: soc.genealogy.medieval
Date: 1999/02/22
Theobald de Valeines (fl. 1135), a tenant of the count of Brittany (honor of Richmond) in Parham, as well as co. Norfolk and Yorkshire, married Helewise -- [15, 3]. He was probably a son of Hamo de Valenis who held Parham as the Domesday tenant of count Alan Rufus [15].
also at a later date, 8 Apr 1999, Raymond Phair posts:
Theobald I was probably a son of Hamo de Valenis; see my 21 Feb 1999 posting about Ranulf de Glanville. Very little is known about Theobald. He occured in the 1130 pipe roll and in 2 charters (about 1135) [1]. The Valognes fee is described by Clay in EYC vol.5, who noted that no connection has been discovered to the Domesday sheriff of Essex, Peter de Valognes, nor any other family using the name Valognes. Theobald's family name was usually spelled Valeines or Velaines.
His grandson, Theobald de Valeines II (son of Robert), founder of Hickling priory in co. Norfolk, justice, and sheriff of Norfolk and Suffolk, died before Michaelmas 1209 when his son Thomas had succeeded [2]. Thomas, according to Dugdale, was one of the barons who rebelled against king John [3]. Clay, Mortimer, and Brown, have discussed other relatives, as well as possible ones, of Theobald I [4].