It seems to me that perhaps we should all step back from "owning" Silbury and concentrate on it as an entirety in its own right. It was built by people 4500 years ago, as a part of their lifestyle. In our lifetime we have recognised it as World Heritage Site and part of an ancestoral landscape that needs to be protected. Its wellbeing therefore comes under all our guardianship, we should be taking the right course of action to repair damage that has been done in the past and not further damaging it to extract some more archaeological information that may titillate a few more books. I don't know how it should be repaired, and I expect there will a lot of people who will have views one way or the other, but its inherent qualities - its centrality in the landscape, its wholeness as a slightly battered but still upstanding beautiful symbol of a peoples desire to create a monument that has lasted all these thousands of years should be respected.
Guardianship is just that, it is the protection of that which is weak and vulnerable, EH or the National Trust, archaeologists, or even the people of Avebury, do not have rights to Silbury, it just needs to be passed on in as good condition as can be expected of something that has lived through the rigours of thousands of years of history, to other generations.
Not on a soapbox, but if everyone put their own interests on one side and just did what was best for Silbury I am sure we would arrive at a SIMPLE solution on which everyone agreed.... of course you could always have a meeting on top of the hill..
Moss