I realised that I never followed up on this thread. It transpired that my a/c was leaking refrigerant and for about 6 weeks after a full recharge it would become useless again. After two recharges with fluorescent dye in the mix, there was absolutely no visible trace of dye anywhere which led both me and the dealer to conclude that the only place refrigerant could be leaking without trace was into the cooling system from the heat exchanger. The dealer replaced the heat exchanger under warranty during the first service and so far, fingers crossed, the a/c has worked as intended. The service was of course at the end of the summer and it hasn't been necessary to cool the car recently, but it would appear that it works ok still several months after the exchanger was exchanged.
