You're right - my suggestion would be simply to impound them. Not return them until their was an assurance of respect. The stone with the flat face toward the camera appears to bear a simple single cupmark. It would be unusual as the cupmark is little known in the south of the country. They're easily overlooked and particularly fresh when they've spent four thousand years buried. Please continue fighting ...

I'll try and capture Bolton's equivalent hill - same size but more oval - on winter sunlit 8mm Kodachrome.

There is a terrible sadness that lies at the heart of all this, the damage that has been done to Silbury over the centuries out of curiosity and the need for treasure, and then the simple truth that what did lie at Silbury's heart was this small mound surrounded by stones with simple offerings.
Taking the subject away from conservation for the moment, and focussing on the spiritual aspect, I believe that archaeologists have to come to terms with a respect for the beliefs of the past. Christianity exists besides many other religious beliefs, secularism strides over christian belief, but it is still respected. Archaeologists have to accept and respect this extra religious dimension when approaching the excavation of prehistoric monuments that obviously have deep symbolic meaning in their past history.
Dean Merewether for all his faults did record what he saw, but he was a man of the church highly intrigued by 'pagan' beliefs, and hardly sympathetic towards them. The stones are part of Silbury therefore should remain inside, they would hardly make an elegant centrepiece in a museum anyway.
The damage cannot be undone and that is what so depressing, but if, as many believe, Silbury has a spiritual meaning, then at least give it back its dignity for a 'remembered' past, and a peaceful future in which there are no further intrusions.