Likeliest place for the leak is the condensor which tends to go porous. The guy who recharged your system should have loaded a dye which shows under UV light so leak can be traced, most pro fridge engineers would have done this but garages dont. The condensor is the radiator type bit of kit up by the radiator itself and involves a complete front end strip down. A friend who is a fridge engineer can arrange this he gets a garage to strip the car he fits the condensor vacs out the system recharges it tests it, the mechanic puts it back together cost is circa £200 ish I think its less but he will tell you for sure. His website is http://www.frosty-wheels.co.uk/ Cheers, Bob