Returned to Walshaw Dean today, I'm now certain it's the circle having been able to recreate the 1902 shot, it's lost some stones, but enough remain & the landscape matches perfectly to confirm that this is the site.
No luck with the grouse butt in the centre theory, I checked out the hill opposite hoping to find dry stone wall constructions similar to that shown in the 1902 shot, but no luck, everything up there is newly built, with no traces of any older structures.