The sort of numbers you are quoting suggests the voltage is simply induced
from the live running adjacent to the earth. I would probably

D'oh! Yes, an obvious contributor. Thanks - I shall pick up on your other
points (common reference via long lead etc.) and start afresh tomorrow. I
tyake your point regarding 1960s wiring and I shall look into the earthing
of the lighting rings.

thanks for the feedback. Time for some sleep!

DDS