Steve - no, it's a romance called Le Roman de Jaufre, written either at the end of the 12th or mid 13th century (opinions vary about dating and whether there was one author or two - anonymous, in either case). There's a relatively recent translation into English available by Ross Arthur, which is just about to come out in affordable paperback (as opposed to the academic hardback version which retails at around £80!). The Lady of the Fountain is in "Yvain, Le Chevalier au Lion" by Chrétien de Troyes as well as in a tale called "Owain, or The Lady of the Fountain" in the Mabinogion. As to the Breton folklore, I've forgotten my original source for that but I'm sure I've seen it in several places.
               
The episode with Morgan le Fay is only one event out of many events in Jaufre, but it's an unusual one for the time. My working assumption is that the version we now have was based on a story in wider oral circulation, possibly as early as the early 12th century, but that isn't going to be capable of proof. I'm working on how to bring it out of academia and back into storytelling repertoire, and I will keep the 'Cat informed of when I'm telling bits of it anywhere. Telling the whole thing would take a few evenings, I think (and that again is something I'll be researching).