A fellow TMer has a Roadster with air con that never worked very well, he spent 2 days insulating the pipes, and modded the foam on the heater intake box to stop hot air coming in from the engine, result was cold air in the footwells. Warning it involves scrapped knuckles and lots of cursing to insulate the pipes. I did the same on my Aero and it made a big difference. I also cover the insulating foam with heat reflective tape, a tricky and time consumming job.
I would take the car to an air con man to get regased, the process does a vacuum test to check for leaks and then it will refill the unit, replacing the old gas.
