"It was originally a straight-sided polyhedron with a domed top (the Saxons probably flattened that to make a fort during the Danish invasions)."

Like to know wot the evidence for a domed top is (didn't you raise the question somewhere else Nigel?). Seems to me if you're going to construct something on the scale of Silbury, with a winding (processional?) path to the summit, you wouldn't want go rolling off it when you reached the top :-) Silbury may have been higher but that's not to say it's not always been flat up there.