Little Hawk, Greg is correct. Most British 'history', c 400-550 is manuscripts compiled by monks about 500 to 600 years after the events described. We rarely know where they got their information. Perhaps a little from Gildas, c 540, and a little from Nennius c 800, but nothing else early. But things are even worse than that, because the manuscripts that comprise the Book of Lafcarn are slanted at best and some are outright fakes. Similarly, John Morris has commented of the 'crazily perverted' Brecon archives. The recent "Journey to Avalon" by Chris Barber and David Pykitt used a genealogy of King Arthur from the Brecon archives to show his patrimony. The two kings prior to Marchall are evidently Visigoths naval commanders that came to Britain rather than go to Spain with the rest in 507 CE when they were chaxed out of Gaul. Marchall, the daughter of Tewdrig (Theodoric) in he archives, is evidently his fellow army commander Marcellus, and not the mother of anyone.

I was fortunate to find some relevant information in Gibbon's 'Decline and Fall of the Roman Empire', because we can take that as real history from near contemporary continental written records, without sifting through person, time, place, and event, in those late British manuscripts with the hope that some of it may be correct.