>Good chance the top was meant to be flat eh?<
Yes, and coupled with the fact that the top of Silbury corresponds closely with the top of the surrounding hills (when viewed from the top of those hills) it is not unreasonable to assume that the top of Silbury was indeed designed to flat. Which brings us back to the point of this thread - ie that if the top was designed to be flat a (spiral) path would have been needed to get there (it's just not possible to go <i>straight</i> up and down Silbury in a dignified manner).
Alternatively, it is possible that this flat top was <i>not</i> used for ceremonial reasons but was made flat in order that it fitted into some greater scheme that included the surrounding landscape. Unless I'm missing something there really are only those two options.