The following is excerpted from Ch. 7 of Mists of Antiquity: "The Misty Origins of the Barclays" http://www.baronage.co.uk/bphtm-02/moa-07a.html#Some Mists Dispersed
Among the scholars painstakingly dispersing those mists which envelop the difficult period immediately before surnames became hereditary has been Beryl Platts, an energetic and indefatigable explorer of ancient charters. In her books on the Flemish Heritage in Scotland she accepted the "family tradition" of the migration from Gloucestershire, on the circumstantial evidence and on the parallels with the migration north of other families of similar ancestry, but she places it as happening in 1124 in the train of Maud, queen of David I, King of Scots, not in 1068 with Queen Margaret.
It was from the manor of Berkeley, in Gloucestershire, of which he was appointed Provost by Earl William fitz Osbern, the Conqueror's kinsman, that Roger de Berkeley took his name. The castle of Berkeley (in whose dungeon Edward II died) was started by his sons and completed by his grandson. But what was their ancestry? Where were the origins of the de Berkeleys in England, the Barclays of that Ilk in Scotland? Mrs Platts demonstrated that before the Manor of Berkeley gave them their new name, they almost certainly bore that of the family of Adeliza, the first wife of their benefactor, Earl William. Adeliza de Tosny had two brothers, Roger and Ralph, (and nephews also who bore those same two names, traditional in the family). Roger and Ralph are the names recorded as those of the builders of Berkeley Castle, and the Tosny family, of the Flemish nobility exiled from Hainault to the Seine valley, were famous castle builders. (Belvoir was another fine example of their work.)
The influence of the Tosny family in England, based substantially on the success of their military doctrine (and contrasting oddly with the later Quaker traditions of their descendants), was immense. Robert de Tosny built the castle at Stafford, another Robert de Tosny held Berkeley in Somerset (different from the one in Gloucestershire held by Roger) of Roger Arundel (son of Roger de Montgomery "Northmannus Northmannorum", whose descendants were to include the Counts of Ponthieu and the Earls of Eglinton), and another Robert de Tosny, the builder of Belvoir Castle, was the senior guardian at Seaton in Rutland of the future Queen of Scotland, Maud de Lens. When Maud went north as queen of David I, a considerable number of Flemings accompanied her at the king's invitation, and among these were the Tosny cadets bearing the Berkeley name and differenced versions of the Berkeley arms.
Mrs Platts proposed a line of descent as follows:
Roger de Berkeley, the first architect of Berkeley Castle, protègé and either brother-in-law or nephew of Earl William fitz Osbern, m Rissa dtr of Agnes, Countess of Ponthieu, by Robert de Montgomery, elder brother of Roger Arundel [see above] and sister of William Talvas, Count of Ponthieu, (whose son Robert went to Scotland with Walter fitz Alan, founder of the Stewarts, acquired the barony of Eaglesham and founded the Scottish line of Montgomery), and had an eldest son, Roger de Berkeley, who continued the building of Berkeley Castle, and at least one other son, John de Berkeley, who went north with Queen Maud.
John de Berkeley had an eldest son, Sir Walter de Berkeley, who m Eva, dtr of Ughtred of Galloway by Gunnilda of Allerdale (granddaughter of Gospatrick, Earl of Northumberland). His dtr Margaret m Sir Alexander de Seton. At this time there appear to be several Berkeleys well established, all of which would be either brothers, sons or nephews of John de Berkeley. Various charters give their names as Walter, William, Humphrey, Robert, Theobald and Richard, and the first two of these held the office of Great Chamberlain of Scotland.
While the continuity of the family is illustrated by the succession of their estates, and of their arms, the exact link between the John de Berkeley who went north with Maud, and the succeeding Chiefs of the Name of Barclay, is yet to be determined with certainty. The Gartly line was reasonably well documented down to the 17th century, and the Barclays of Mathers and Urie, of which the founders of Barclays Bank are cadets, down to the present day. On the expiry of the Gartly line the representation of the family passed to the Barclays of Tollie, and the Chief of the Name, Peter Charles Barclay of Towie Barclay and of that Ilk, matriculated his arms in 1971. (The consolidated history of the Barclays will appear eventually in Moncreiffe's Family Records.)