moss ;"I thought of this legend,but couldn't decide if what I remembered was from The Crystal Cave by Margaret Stewart, and so part fiction. What about the legend of Bran the Blessed, his head is buried at White Mount and if it ever leaves off its placement there, guarding our shores, Britain will fall. (Bran means raven - which gave rise to the obfuscation that Britain would fall if the ravens left the Tower of London at White Mount). That aside, the head and the stone are closely connected in celtic myth and legend."
As Gladman says Welsh history is extrordinary complicated without bringing in Nennius, Geoffrey of Monmouth and Gildas the monk into the discussion;)
As you say Cunedda migrated down from Scotland and came after Vortigen as a leader, interestingly if you read the Vortigen study link on the Dinas Emrys page, the sacrifical boy/magician/next leader who told the tale of the two dragons - Ambrosius, may be part of the legend of Stonehenge as well, a meeting having take place at Amesbury.
Whatever this 'dark age' when all was up for grabs as far as storytelling is concerned had I believe the one mention of Arthur up in Scotland fighting a battle, now whether he came down with Cunedda might explain his presence, I like to think Arthur was a king at Caerwent, but Tintagel is another stronghold for him.
Trying to remember the story of the warriors/king who slept in a cave, and if you woke them up, he would become the next king of Wales? or have I got it wrong..