Rare - its out of context for a start
Or In Context if you're a believer!

"Silbury is a one-off"
or Not if you're a believer!

As you say, only excavation will settle it.
And even then, it'll need publication, as SL says.

Incidentally I don't see how "being at the bottom of a hill next to marshy and presumably boggy ground" but clearly built straight off the solid chalk spur of Waden makes it significantly different from Silbury.