I've just realised 'which' Maes Knoll you are refering too. It is the one that joins on to the Wansdyke near Norton Malreward, Bristol, and the Wansdyke is the very same one that JC was refering to.

I honestly think this is what it seems ... a promentary fort and nothing more. It may well have been placed on an older structure. It may well have been modelled on Silbury.

Has anybody ever conjectured that Silbury was just the base to a fort that was never finished?

I can go along with that.
I was born near there (although some say it was outside of the village of wedlock :) and know the area well.
I have noticed that the hill also looks like waden hill, same size and shape.
I have some pano's on my site @
http://www.users.myisp.co.uk/~gtour/frameset.html
follow the More Circles link.
The large circle at Stanton drew is the same size as the inner circles of Avebury. There was a chambered tomb nearby at Nempnett Thrubwell (great name!) which locals called the fairy toot.
It had 16 chambers which would have made it the largest in the country.
It totaly destroyed now :(
Pete G