I'm loving your work Livingrocks. Think I might have to take a shufty up there tomorrow, but I am confident that your picture is the same thing as the old photo. I wonder where the central feature has gone?
I reckon the central feature was probably a grouse butt, and not being earthfast was probably used in the reservoir lining - it had apparently gone when Ling Roth saw the circle in 1906.
The circle is dead easy to find, walk up to the Lodge look over the dam and it's right there, only a few metres from the dam wall.