So we form ourselves into the Silbury Leaseholders Association, get ourselves a legal identity (very cheap) and write to Lord Avebury asking him to sublet it to us for 10 years at a pound a year, on a lease that conferred zero rights to us.
It's worth a try. It's worth nothing to him because he can never use it to generate an income. All he gets is hassle from EH, and he's virtually said so. If we were leaseholders they'd be legally obliged to talk to us about stuff, just as much as to him. And we could say things to them that he clearly would like to say himself.
(Also, although we'd have given ourselves no rights of access to the Hill, we'd be perfectly entitled to electrify our fence....)