Whoa! That's a weird one Celtic. Let's see . . . . . . . . The meters cycle up and down as the platter rotates, and the arm is on its rest correct?
Well, the only way the meters are getting a signal is from the phono cartridge (and not from mechanical motor vibration unless it could somehow be transmitted to the cantilever) so something in/about the rotating platter is generating a low level electrical current in the resting cartridge.
I forget how Rega platters are built. Is it metal (or have a metal sub-platter?) Or is it non metallic but has metal weights countersunk into it from underneath? What I'm driving at is the platter (or parts of it) may have acquired a magnetic polarity from sitting in the same place or position for so long.
See if you can borrow another Rega platter just to compare it to yours. You can always demag your platter using a bulk tape demag unit.
This thread is starting to sound a bit like "Car Talk" ha, ha!