http://www.medievalgenealogy.org.uk/cp/p_willoughbydeeresby.shtml#p658
Volume 12, part 2, page 658:
He [Robert de Willoughby (d. 1317)] m., in or before 1303, Margaret, da. of Sir Edmund (DEINCOURT), 1st LORD DEINCOURT, by Isabel, da. of Sir Reynold DE MOHUN, of Dunster, Somerset.
According to Complete Peerage, vol.4, p.120, Isabel, the wife of Sir Edmund Deincourt, was the daughter of Sir Reynold Mohun by his wife Isabel, daughter of William (de Ferrers), earl of Derby. But the issue of Isabel de Ferrers had certainly failed by April 1327, when an inquisition was taken after the death of John Meriet, who had married her granddaughter. At this time Margaret was still living. Clearly, either she was the daughter of another wife of Edmund Deincourt, or Edmund's wife Isabel was not the daughter of Isabel de Ferrers.