That is of course, if it was around at the time, and is not the remains from the digging of the roman road.... ;) but I think Bradley took his notion from Pollard's drawing of the sacred landscape round Silbury (its in Writtle's book), which gives prominence to the palisaded enclosures between The Sanctuary, Silbury Hill and WKLB...
The function of Silbury Hill after it was built was to provide a platform on which was enacted- whatever... night fires, music, or ceremonies, it respected the older sacred places that surrounded it but was not necessarily tied into sun/moon ceremonies, it was just a part of the evolving religious mythology....