IMHO there wasn't one.
I sent Wally some pics to help with his 3D model and altho it looks good the reality isn't there.
A spiral path would be much more evident at the bottom as the feet trampled the path over the years.
The only evidence of this spiral path is that you can walk from the top of Silbury and follow a path for 1 turn and a bit before it stops. Dave Field demonstrated this to a local TV crew a while back.
This Must be the spoil heap that Drax threw over the edge when excavating the bore tunnel in the 1770's
It is the only place for the spoil to be (now we know that its missing.)
As the old hill has been knocked about by the Saxons and the people who came before only further investigations (none intrusive hopefully) could tell us what the original angle/shape of the mound was.
The excavations at the top do appear to show that the top was flat because of the chalk-stone work that was found when trenches were dug in the 1960's and a few years ago,
But that does not mean that it was an observation point that had to be climbed (processionaly or otherwise.)