Ooh, that's interesting. It's very angular and sharp at the edges, I assume it's a limestone slab so you might expect more erosion if it was Bronze Age, but it's certainly not impossible that it's a fallen stone which has been re-erected. Is it on any boundaries (estate, parish, etc)? There are a few smaller standing stones on Minchinhampton Common that are probably medieval or post-medieval boundary stones of some kind.