"I've never assumed its natural because of the stukeley sketch."
That's fair enough. It's excellent to have some (fairly*) reliable old evidence.
Even if it is totally natural it doesn't necessarily detract anything from it. In fact, to me, it actually adds to its importance, as this would mean that Silbury was based upon it, ruling out the possibility that it is a pale immitation of Silbury.
* I say fairly only because of lack of precision etc.
