Oooh, that's a nifty tool, cheers. There's my evening ruined :)
Thought about the field boundary option, there are apparently the remains of Iron Age and later fields in the immediate area underlying the current field patterns, but to my eyes it just looks a little too regular considering the layout of the current field boundaries, and why rounded corners?. Anyhoo, I've still found nowt about it on the web, so maybe it's just a happy coincidence fer that years crop. The site is a fairly flat plateau, I'd love it to be a rediscovered marching camp or fortlet, or a later Saxon burh, but it'll probably amount to nowt more than an geological anomalie.