Well, I don't think I did!! I spend over an hour wandering the plateau but couldn't find anything resembling the ring cairn/cairn circle that Burl mentions. Much frustration and a look at Coflein later, I think the cairn circle and the associated cairn and fallen stone are actually further to the west, nearer to the cwm.
I came from the west (Mynydd Bwllfa), but crossing the cwm turned out to be more difficult than I thought, meaning that I emerged next to the biggest (SW) clearance cairn but missed the real thing. The annoying thing is that the cairns marked on the OS are the clearance ones, the "Rhondda Stonehenge" itself isn't marked at all. Bah!
Update: This isn't it. The circle is exactly where shown on the Coflein map extract, but is completely unmarked on the OS map and I was in the wrong place on this first visit. The cairn in this picture is one of the ones in the cairnfield to the NE of the circle.