That was what i first thought but I've not come across any Derbyshire old barns or farm buildings constructed like that, and Ive seen a few, so the only other thing I can come up with is a Roman hypocaust or something medieval like dog kennels? Its obviously old, but how old?

I would guess it measure about 15ft across (5 metres) but its hard to tell. The site has with a few recumbent stones laying around as well.