The milestone is near the pub on the Rodborough Road (The Bull?) and can be visited along with Whitfield's Tump long barrow on Minchinhampton Common. There is a very similar milestone near the long barrow, the back of which is covered in small circular depressions:
http://www.themodernantiquarian.com/post/78607/whitfields_tump.html
The Rodborough milestone has some little fossils embedded in it, just for a bit of extra interest.
There is another broken milestone a bit further along the road as well. These are the ones that are mentioned in the Sulllivan book.
The other Horestone is in someone's garden I believe. It may be prehistoric, but the other's aren't.
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As an aside, Sullivan's book also mentions two little stones at the foot of Tom Long's Post on Minchinhampton common. One has a red reflector mounted on it. Contrary to the suggestion that they may be part of a vanished long barrow, they are modern. A photo of Tom Long's Post from the 1930s shows they were not there:
http://www.stroudphotos.co.uk/page7.htm
But by the 1960s: