Thanks for all the suggestions. The new bike is obviously the best suggestion, if the least parsimonious. But i've dug out an old compass and i'll investigate the scene of the crime and report back. Can't find a map of any installations that throws any light.
Edit: Well, I'm none the wiser. Comp zeroed out a bit further down the hill than usual then recovered. I got off and walked up and down the pavement like a demented water-diviner, but did not register anything of note on the compass. No nearby overhead power lines or sinister-looking government buildings - just a cycle shop and a vet.