I realize that I forgot poor old William, the Breton author of "St. Goeznovius," (1019) who wrote:

"In the course of time, the usurping king Vortigern, to buttress the defence of the kingdom of Great Britain which he unrighteously held, summoned warlike men from the land of Saxony and made them his allies in the kingdom. Since they were pagans and of devilish character, lusting by their nature to shed human blood, they drew many evils upon the Britons.

Presently their pride was checked for a while through the great Arthur, king of the Britons. They were largely cleared from the island and reduced to subjection. But when this same Arthur, after many victories which he won gloriously in Britain and in Gaul, was summoned at last from human activity, the way was open for the Saxons to go again into the island, and there was great oppression of the Britons, destruction of churches and persecution of saints. This persecution went on through the times of many kings, Saxons and Britons striving back and forth...

In those days, many holy men gave themselves up to martyrdom; others, in conformity to the Gsopel, left the greater Britain which is now the Saxon's homeland, and sailed across to the lesser Britain (Brittany)."

William claimed a source called "Ystoria Britanica," which could well have been the same book used by Geoffrey (William was Breton, and Geoffrey seems to have been half Breton, and "The British language" could mean either Welsh or Breton in Geoffrey's claims about the source). Since William was just providing background to his story of Goeznovius, he would not go into any detail about Arthur, even if that detail were there...but again, a pretty clear source in the early eleventh century, implying a tenth century origin for HIS source at least.