Afternoon,
I have never seen tjhe circle actually in situ, but funnily enough I was there a week or two ago (wearing my Heritage Action hat). I took a picture from the exact position that I think that drawing was sketched from (I got it back today), and we all know what has happened to that field. Now, there's a field behind it (assuming you are facing the house down in the valley) that has a circular embankment in it, it's on the crest of the hill. It could be a mortuary site or a camp, who knows. It is certainly worth having another rummage around up there, you never know what you might find.

Dave