just a quick update: I used an implement to apply sideways force to the data wheel (jiggled it) and it has started to work normally !
It seems to me like either a dry joint on the pot underneath, or some sort of mechanical problem.
I'm going to leave it for now. If the fault returns I'll attempt to re-solder the pot joints after I learn how to take the tyros to bits !
Thanks again for all replies.
Simplorer