We could take our first step towards archaeo-terrorism, in a Guardian-reader sort of way, by fixing our own warning notices to Silbury's fence, to replace the ineffectual EH ones. It might reduce the numbers of trespassers a bit, which would at least be something concrete rather than spending all our time in talking mode.
Perhaps we should make it a point of honour for TMAers to take a notice with them when visiting there and to post a picture of it here.
It's a nice legal point as to whether we'd be doing anything wrong.
I like dual use efforts, so I reckon the wording ought to be always the same, something like "Danger - Keep Out - this hill has had no maintenance for 3.5 years".
I wonder what they'd do? Is it in the public interest to pull down warning notices?