Picked up this bit of information from a library book today;
taken from The Secrets of the Avebury Stones by Terence Meaden;

"a kilometre south of Silbury, is a sarsen site, now empty, on higher land towards Allington Down "(SU 0985 6714).. Stukeley named it South Downs Barrow and said it looked like a longbarrow,"made only of stones pitch'd in ground". part of it still seemed to be around in 1885..
Measden goes on to say that the Kennet flows on to be joined by the intermittent waters of Waden Spring, passing e/w through the middle of a double ringed late Neolithic palisaded enclosure (SU112 682) at West Kennet (excavated by Whittle). There are circles of wooden posts, and also a second timber structure that stood close by, its longest axis (it was oval in shape) directed towards Silbury hill; the theory propounded that these sacred circles and structures were "perhaps part of the process of labour mobilisation for the monumental undertaking of the mound".

The other point relates to Swallowhead Springs, and is interesting because it relates to Bath's own Celtic Goddess, Sulis. He points out that Swallowhead is phonetically similar to Suilohead, and may come from the same root as Sul, so this may be a lost name for the Goddess of Silbury hill; though I think Tombo has said this already.
Sul derives from the Celtic for 'eye'(suil) and 'seeing'.. the winter waters of the sacred Suilohead spring provided the sacred link with Silbury, which in wet winters is encircled with water.
Moss