Weis' "Ancestral Roots. . ." (39:29).
No parents for Margaret here. For Roger's parents I have Alan Ceoche/la Coche (a.k.a. la Zouche), d. 1190, and Alice, d/o Philip de Belmeis and Maude de Meschines/le Meschin (d/o William le Meschin). And there appears to be a line here going back some nine additional generations to Fulk the Red, Count of Anjou (d. 942).
Roger la Zusche who, for his fidelity to King John, had a grant from that monarch of the manors of Petersfield and Maple Durham, co. Southampton, part of the lands of Geffrey de Mandeville, one of the rebellious barons then in arms. In the next reign he was sheriff of Devonshire and had further grants from the crown. By Margaret, his wife, he had issue, Alan, his successor, and William, who left an only dau., Joice, who m. Robert Mortimer, of Richard's Castle, and had issue, Hugh Mortimer, summoned to parliament as Lord Mortimer, of Richard's Castle; and William Mortimer, who assumed the surname of Zouche, and was summoned to parliament as Lord Zouche, of Mortimer. He was s. by his elder son, Sir Alan la Zouche. [Sir Bernard Burke, Dormant, Abeyant, Forfeited, and Extinct Peerages, Burke's Peerage, Ltd., London, 1883, p. 598, Zouche, Baron Zouche, of Ashby, co. Leicester]
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Ancestral Roots, p. 43, younger son (of Alan Ceoche of La Coche), heir to brother William 1199, sheriff of Devonshire 1228-31, a witness to Henry III's confirmation of the Magna Carta, d. shortly before 14 May 1238. Browning, p. 308, lists him as son of Roger, son of Alain IV, Viscount de Rohan, Count of Brittany and Mabilla, dau. of Raoul II, Lord of Fourgeres.
[http://library.monterey.edu/merrill/family/dorsett6/d0005/I10853.htm]