The site you've listed as Cerrig Duon stone circle at SN851206 is that in fact the Maen Mawr stone and stone circles on the bank of River Tawe ?
The grid ref. match with this site but I've never known it under the name Cerrig Duon.
Best Wishes
Joann Hanmer
Something is not right. This message is just to keep things from messing up down the road