"We tend to think we "know"what our ancestors wanted/beieved."

Actually, isn't that the whole issue - not just with Time Capsules but offerings. We really DON'T know what our ancestors wanted/believed. That being so, the only "safe" thing to do is nothing.

But its not really for us here, or EH or the Avebury parents to decide. Silbury is a World Heritage Monument "of universal cultural value to humanity". How it should be treated is governed directly and indirectly by a whole bookshelf full of international treaties, conventions, conferences and the like. Tens of thousands of fancy words, but if you care to have a look at them they ALL actually simply say "if in doubt, do nowt".

EH should read them, not try to tell us there are "many conflicting conservation issues" here. When they finally do, they'll see. I doubt if they'll go ahead in the end.