Call me the sceptic, but isn't the high radiation just due to the underlying granite bedrock, and the vast amounts of radon produced by it? After all, cornwall is one of a few regions in Britain with ultra high radon readings.
I wouldn't dare suggest that anything 'spooky' is going on at these places. And yes, the granite/radon thing is probably the cause for these effects, but I can imagine that the geometry of Man-An-Tol's 'doughnut' may concentrate it further.
Let's all suggest it was some ancient radiotherapy device built buy super intelligent beings that have long since left the planet.