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?