The myth of "Isabella/Elizabeth Plantagenet" as wife of don Sancho Martinez de Leiva in Spanish Navarre appears to be of relatively late origin. It is recorded in a couple of 18th century works purporting to be historical (Thomas Carte, A General History of England, Vol. 2 (London, 1750), and John Talbot Dillon, The history of the reign of Peter the Cruel, king of Castile and Leon, Vol. 2 (London, 1788) - but the latter in particular is full of egregious errors.
"Isabella" was supposedly the daughter of Edward III and "Elizabeth Suffolk, Countess of Northumberland" - which rules the claim out of court immediately because there *were no* Earls of Northumberland during the reign of Edward III. (The first Earl of Northumberland was Henry Percy (1341-1408), and the title was granted by Richard II, *grandson* of Edward III.
There was no "Suffolk" family.
The Uffords were Earls of Suffolk during the reign of Edward III, but they had no daughter Elizabeth or Isabella. http://fmg.ac/Projects/MedLands/ENGLISH%20NOBILITY%20MEDIEVAL2.htm#...
While there appears to have been a historical Sancho Martinez de Leiva, it is not clear whether he participated in the battle of Poitiers - the English allies were Gascon and Breton lords, not Navarrese. Sources other than the gossiping Englishmen give his wife's name as Aldonza Lopez de Avellaneda. https://it.wikipedia.org/wiki/De_Leyva (in Italian)
The whole legend is possibly a grotesquely distorted echo of Enguerrand VII de Coucy and Isabella, eldest *legitimate* daughter of Edward III. Enguerrand *did* meet King Edward in 1359 (as a hostage during the negotiations for the release of the King of France, captured at the battle of Poitiers), found favor with him, and attracted the attention of Isabella, who had previously endured - and caused - several failed betrothals. This time she made up her mind that she would have Enguerrand, and her father agreed. They were married in 1365.
