Stu's OS co-ordinates are spot on so I'm suprised you couldn't find it although I would reckon it's about 50 metres rather than 100 metres from the cairn. The grass is still really short too so it's worth getting back up there to have another look but watch out for the stag with his two doe's that I spotted - he had a hefty set of antlers on him and is probably best avoided.
I'll post some pictures of the circle tomorrow that might help you find it in case you can't take megadread up on his offer.

-Chris