When I had my first MX-5, because it was an '89 it needed some sort of special gas that was no longer available to be regassed (banned for CFCs or something
).
I would have had to drive to some specialist in North Yorkshire to have it done... never did in the end, and haven't had a car with aircon since! I gather if you are paying to have it regassed somewhere decent they will probably clean out the filters, etc too?