In the book Mysterious Britain by Janet and Colin Bord, the tale of the druid offering a seemingly bottomless cup to passersby is mentioned under the section discussing the nearby Cheesewring rock formation. However instead of naming this Barrow Rillaton, it is named Arthur’s Grave cairn. I’m of suspect this is an error on the authors part as I have yet to find any other sources that give it this name. The story is vastly the same as the one other site users have attached to this site, though it gives the story a possible link to the grail quest of Arthurian lore. The early iterations of the grail story can be linked back to the Welsh text known as the mabinogion, in this the grail is a cauldron, and many various spring up that have abilities akin to the grail, one story is of the vessel being unable to be emptied/ bottomless. This was so as to prevent starvation/ it could sustain all to give life everlasting. Perhaps grasping at straws with this one but I’m also posting this here in the wonder if someone else might have heard of this site under that name.