"Lets just fix the hill. If later, a solid long term research project decides that an intrusion is required then so be it - but remember, it won't be long now before we will be experimenting with robotic archaeology - such a thing could allow for much smaller bore entrance tunnels and so seriously limit any impact."

This makes a great deal of sense and should be an argument that every archaeologist (without personal motivation) can support. Sites are often left unexcavated or back filled so that future archaeologists with future technology will be able to investigate later. Just compare that with the development of keyhole surgery - the patients live with keyhole surgery and previously died from massive trauma when more intrusive surgery was the only option. We are in danger of killing the Hill by using crude surgery now rather than using keyhole tomorrow.

We don't have to bulldoze into the hill now with crude tunnels just to get at undisturbed Neolithic ground. Is it really going to decay and be compromised that quickly? Is the ground beneath the hill MORE IMPORTANT than the hill itself?

I also firmly believe that Silbury Hill belongs to the people of Britain rather than to archaeologists and EH, but public opinion is dangerous because it can be manipulated. Many will share sbp's view that it is just "a pile of mud". Of those people that will care at all or have any opinion, many will like the idea of tunnels especially if there is a hint of "solving the mystery", "buried treasure" or even "the burial chamber of Britain's King Tut". Spin doctors will make it attractive if vested interest and personal ambitions want it to go that way.