The credible Jacqueline Simpson together with S. Roud, in The Oxford Dictionary of English Folklore (2000), addresses this topic: They maintain that "There are no old folk traditions about the Holy Grail, though it is prominent in French Arthurian literature from the late 12th century onwards...Although the story suited medieval piety, Church writers never adopted it...Some scholars have argued that Celtic myths about magic cauldrons underlie the medieval texts." Simpson and Roud, unfortunately, do not take a position on whether they agree with "some scholars," almost certainly because this is a fanciful idea that can be neither proven nor disproven. For myself, I cannot see Chrétien de Troyes inventing it wholesale. It smells of a folk tradition behind the literature, but whether or not that tradition had deep roots remains as unanswerable as is the question of Chrétien de Troyes inspiration. Literary speculation, not historical process, rules this roost.