Long barrows.. this is very true.
Only one thing for it then. We'll have to find out even if it mean flattening the whole thing! a la Marden.
But they do know it's neolithic? because of the organic finds? and the northern neolithic round mounds like Willy Howe are burial=less. It would be nicer to think that such a huge undertaking as Silbury wasn't based on the egotism of one person, wouldn't it.
(that must be my unsquashable desire to see neolithic times as the idealistic home of the ecologically sound, nice to other people, universe appreciating noble savage. I know it's undoubtedly untrue, but it's a hard one to shift)