The stepped pyramid concept has always seemed the most likely to me and I seem to remember that an artist's impression of such was on display when I was last there. Since then, I seem to recall that theory having been called into question. Spiral paths at Silbury, Glastonbury Tor and Merlin's Mount do not convince me and seem to be wishful thinking combined with (in the case of Merlin's Motte) later landscaping. I do agree though, that Silbury would have been a shining white pyramid in its day and probaby would have been kept that way by regular scouring until the local religion went off in a different direction.