"Incidentally I don't see how "being at the bottom of a hill next to marshy and presumably boggy ground" but clearly built straight off the solid chalk spur of Waden makes it significantly different from Silbury."
Thats if Silbaby WAS on a chalk spur, we don't know that unless its excavated.. If it is ever excavated bet you £10 that there is'nt a barrow ;)