I think we must have visited Gatcombe Lodge too yesterday; all down to friend with map – I can take no credit apart from tagging along gamely. Just across the road from the Minchinhampton Long Stone there is a tree covered long barrow, and yes we were passed by a couple of landrovers which we, perhaps naively, ignored. From here we continued into the estate, no-one challenged us; we walked up (and down) some steep slopes to eventually reach Hazel Wood in search of another long barrow. According to the map this barrow is in a field at the top of the wood – which had restricted access. Couldn’t make out more than just a raised curve to the land so must be mostly ploughed out.
The route we took was marked on OS Explorer 168 as a footpath (apart from the Gatcombe Park bit) so I guess it was ok to walk there.