To my mind the case put forward here by Rev AC Smith for Silbury being sepulchral, i.e. being built to contain a (as yet undiscovered) burial, is ever so convincing.
http://books.google.co.uk/books?id=KDYGAAAAQAAJ&dq=The+Wiltshire+Archaeological+and+Natural+History+Magazine+By+Edward+Hungerford+Goddard&pg=PA1&ots=2QvYknPk0s&sig=9RzuoCa7-9VydT1MBo8BVWO5Q_k&prev=http://www.google.co.uk/search%3Fhl%3Den%26q%3DThe%2BWiltshire%2BArchaeological%2Band%2BNatural%2BHistory%2BMagazine%2BBy%2BEdward%2BHungerford%2BGoddard%26btnG%3DGoogle%2BSearch%26meta%3D&sa=X&oi=print&ct=result&cd=1#PPA144-IA2,M1
(Page 144 onwards)
The general presumption that it is a very exceptional and puzzling non-sepulchral monument arises from an all-too-facile assumption that the body would have been in the centre, but he offers extremely good reasons for thinking it may equally well not have been.
EH have now almost finished excavating and haven't found a burial, which is hardly surprising since they have broadly kept to the areas already investigated. But Rev Smith's reasoning remains just as strong, what hasn't been found may well still be elsewhere, undiscovered. On this basis, EH's starting ambition to "finally" discover the true nature of Silbury can hardly be said to be fulfilled and was never going to be.
"King Sil" might be sleeping peacefully just ten yards from the central excavation or be doubly and irrevocably entombed within the collapsed material. So hopefully, assuming no-one ever decides to tunnel into it again, the true purpose of Silbury will remain an eternal mystery. Good!