Thanks, everybody, this is very informative. One tends to overlook the importance of the Romans in Britain from a religious point of view.

Marsaili, I'm sorry I can't help you with your queries, though I can see how the seasonal adjustment would have been a concern for monasteries. About lithurgy, I might have come across something on the Net while looking for texts but I can't remember now. If I find it again I'll be sure to post it.