My original post now edited as I read the MP article at speed and missed the fact that there are two stones in question - the one mentioned by Sullivan and another found by Stephen Davis and Clare Forbes. The article needs careful reading - here is an extract:

The Sullivan Horestone is on the West side of the road between Minchinhamton and Rodborough by Ridgmont House - viz at the point where the road crosses from Minchinhampton into Rodborough (just S of the Bear Inn) -
The Davis/Forbes Horestone is in Bownham Park and more specifically to the right of the rear driveway to Bownham House (behind a very large tree up a very short access to a much more modern house) - just before you reach the Minch-Rod boundary (going N) turn right into Bownham Park - after 50m you reach a T junction (with the front gates to Bownham House facing you) turn left into the more modern estate and follow the road downhill - almost at the bottom of the hill there is a right turn (in fact the 2nd rt) into what seems to be into a wood by the side of a modern house - park in the hammerhead (about 20m up the access) - The Horestone is immediately to your left by the iron railing fence dividing the modern house from the rear driveway of Bownham House -

PS: Thanks for you comment Drew ;0)