[Arran]

Mike;
  I can only take a guess but it sounds like the tuning capacitor might be seized up, but I too will await clarification from the original poster to find out if that is what he meant. Short of severe corrosion or mechanical damage there isn't really a lot that can go wrong with a tuning cap, usually it's a matter of cleaning and new lubrication in the bearing points.
Regards
Arran