|SOME NOTES ON MEDIEVAL ENGLISH GENEALOGY|
Volume 1, page 22:
He [Reynold de Briouze (d. 1227 or 1228)] m., 2ndly, 1215, Gwladus Du, da. of Llewelyn ap Iorwerth, Prince of North Wales, by his 2nd wife, Joan, illegit. da. of King John.
The identity of Gwladus's mother is still a matter of debate. There is no clear contemporary evidence that her mother was Joan, and a daughter of Llewelyn and Joan could not have been aged more than about 9 in 1215. The only direct statements appear to come some centuries later. One strand of the later tradition does identify Gwladus's mother as Joan, but another says she was Llewelyn's mistress Tangwystl.
[This question has been discussed by - among many others - Stewart Baldwin, Rosie Bevan, Ken Finton, Todd Farmerie, John P. Ravilious, Paul Reed, Douglas Richardson, Henry Sutliff, Nat Taylor and Brad Verity.]