The thing that bothers me about a 'tip' is you can't actually do anything on it - Silbury really does become something like an Egyptian pyramid (in function). In one of the news reports or films (sorry, can't remember which one) someone states that there is now evidence of a spiral path winding its way round the structure; along with the causeway from the road that indicates there was a way to the top and that something went on up there (unless we accept that the spiral path and causeway were simply to aid the building of the structure and were then covered over/removed.

Dunno WA, just something about the way Silbury looks today (give or take a few meters) that looks right.