The Stones of Stonehenge team will be doing some non-invasive geophysics at the Penlan (or Troedyrhiw) stone pair during the coming fortnight, in the hope that the stones might be the last remnants of a stone circle. Hmmm..... Anyway, I offer this as something that might be worth a look. I think I see (on the Bing satellite image) the traces of an embanked circle about 200m due east of the standing stone pair, in rough ground on the other side of the road leading to Carnedd Meibion Owen. Grid ref SN09226 35713. A bank has been cut across it.
If there is a circle there, its diameter is about 50m. I haven't been able to do a comprehensive search, but I'm not aware of anything in the Coflein or other records.
In my view it's worth taking a look.......
The Possible circular structure is slap in the centre of this image.