Hal Bradley, an excellent compiler, has no name for her:
https://freepages.rootsweb.com/~hwbradley/genealogy/aqwg1357.htm#22361
Sir Geoffrey de DINHAM Knight
1Weis, Frederick Lewis, Ancestral Roots of Certain American Colonists Who Came to America before 1700 (7th ed., Baltimore: Genealogical Publishing Co., 1992.), pp. 214-29, Los Angeles Public Library, Gen 974 W426 1992.
2Cokayne, George Edward, The Complete Peerage of England, Scotland, Ireland, Great Britain and the United Kingdom, Extant, Extinct, or Dormant (London: St. Catherine Press, 1910.), 4:369, Los Angeles Public Library, 929.721 C682.
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Phillip Lee Gibbs Has found a citation for her:
www.celtic-casimir.com
Sarah DE HORNACOTE
Born: 1218, Tresolyth, Cornwall
General Notes: daughter and co-heir and eventually heir to Margery her niece, Assize Roll 30 Edw I.
Events: 1. Alt. Birth; 1214, Hertland, Devonshire, England.
Marriage Information: Sarah married Roger CARMYNOWE, son of Robert DE CARMYNOWE and Unknown. (Roger CARMYNOWE was born in 1214 in Tresolyth, Cornwall.)
Marriage Information: Sarah also married Sir Geoffrey DE DINHAM of Hartland, son of Oliver DE DINHAM & Hartland and Elizabeth. (Sir Geoffrey DE DINHAM of Hartland was born in 1200 in Hertland, Devonshire, England and died before 26 Dec 1258 in Buckland Dinham, Somerset, England .)
The Carminowe Family
Roger Carminowe married Sara Hornicote, daughter and co-heir of Gervas de Tintagel or de Hornicote. Henry II granted the whole County of Cornwall to Henry Fitzcount who granted Tintagel (the traditional birthplace of King Arthur) and Hornicote to Gervas, who left them in 1208 to Robert Tintagel. King John, who had confirmed all the Fitzcount grants in fee in 1215-16, in 1220 ordered their resumption into the King's hands, except the lands of Robert de Tintagel. According to tradition Tristram and Isolde were buried at Tintagel
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Celtic Casimir is a reasonable on line tree but doesn’t cite sources.
Please help find more sourcing.