MJB, thanks for the book reference, I'll follow it up.
The site is well worth a visit, if you've got the energy. The obvious and easiest way to create a bank up on Carrock Fell is by doing exactly what they did - use the rocks on the summit. Digging any ditch would have immediately unearthed boulders. I did get an impression that the site could be Neolithic, but information is scarce.