Steve,
There are lots of possibilities - some more expensive than others.
It seems a fairly common problem for the system to leak gas over a period of time - particularly if you don't use the aircon system regularly. The old R12 compatible hoses have a bit of a reputation for leakage and corrosion can play it's part too - the condenser, dryer, expansion valve and evaporator all suffer and can eventually need replacement.
The thing to do is to have the system pressure tested (usually done with nitrogen) and if it holds a reasonable pressure for 30 minutes to an hour, have the system regassed and make sure they inject a dye so any leaks can be seen with an ultra violet light. If the system won't hold the pressure you should be able to get a clue about the leak from the hissing sound you'll hear.
Costs depend greatly on which bit leaks. Most common is the condenser - because it's up front and can get damaged by debris. Most expensive is the evaporator - not only because it's £500+ for the part but it's at least that again for the labour to fit it. Usually if the evaporator is changed they will most likely do the expansion valve too (£99). The compressor seems pretty good although I've heard of bearings wearing out.
If you have it repaired it will probably have to be converted to R134a anyway since R12 is now illegal but that won't add much to the expense. It may have already been done anyway if it's recently been regassed.
The aircon in the 964 wasn't the greatest although it's certainly better than none.
Hope that helps.
Regards
Dave
BTW I had my evaporator/expansion valve done last year so I know what you're going through!!
