Hi Moss,
In Bradleys latest book, The Prehistory of Britain and Ireland, he puts forward the theory that Silbury and other mounds 'might be considered as raised platforms which would provide a vantage point from which a small number of people could observe activities that were otherwise concealed', i.e. a view into the West Kennet palisaded enclosure. He stites the Conqueror barrow on the edge of the henge at Mount Pleasant and the raised area on top of the largest chambered tomb at Newgrange as other examples.