interesting web-site, thank you. I only read the religion page but I'll try to read more later. I think he has made too much of a rivalry between the Celtic church and the "Roman" church. Maybe he has embellished it for a little more intrigue?
The missal used by the Celts at that time includes prayers for the Pope in the Canon of their Mass. It's doubtful they would be praying for the Pope's intentions if they were bitter rivals with "Rome".
Stowe Missal -
http://en.wikipedia.org/wiki/Stowe_Missal