Don't forget though you are on private land, weedkiller is not an option, animals could eat the poisoned stuff.. Gardener or Guardian is the question?
I suspect a lot depends on the farmer and his attitude to a) a lump of stones and all those pesky people who keep walking across his land to view it ;).... or b) a megalithic legacy to be protected for the enjoyment of the public. For instance, do we all have a right to take a photograph of every prehistoric stone?