It has been suggested to me that maybe we are looking at this all wrong. Instead of it being a smaller silbury maybe Silbury is a huge emulation of the smaller mound with its semi natural features of a spring and moat.
PeteG
Dating the mound shouldn't be too difficult but I tend to agree that we've got a Silbury prototype here rather than a later scaled down version (why build a smaller version of St Pauls when the original already exists across the road?). Unless, that is, Silbury Hill and the Silbury Mound are contempory and form part of an already huge, egineered lanscape - the mind boggles.