In case anyone thinks Pete and Jim have gone nuts, can I just add confirmation.
Park at WKLB carpark, walk to the bridge, turn left and walk a couple of hundred yards and look back at the road, which is high up at that point. It's there, hard up against the bank, with trees and scrubby bushes on it and with the rear bit cut away but unmistakable.
OK, I presume it's known about but not by the general public, which is absolutely extraordinary.
Why? Well imagine a scaled down Silbury, cut off to a flat platform one third of the way up, with a circular moat round the front half of it full of clear water in which you can see the spring bubbling up. Imagine it has sloping sides at the same angle as Silbury and adjoining it is a dead flat flood plain of the Kennet, just like Silbury.
Imagine this, the third largest construction in the Avebury landscape, just sitting there, unregarded by the thousands of people walking the path up to WKLB...
This is just bizarre. I recommend everyone goes and looks urgently, in case all three of us are collectively mad...
I have some really good photos but can't post them until a name is found.