Thanks Stoney. Its going in for two days next week to try and trace the fault. I'll mention this to him. The air con has always been a weak point on this car.
The garage have found no faults - althought they have apparently contacted Ford and thay have said that the air con system does not function if the outside temp is below 10 degrees C. I have found reference elsewhere that it does not function if below 4 degrees C! I guess I will have to wait for a warmer day to check if this is right. My garage have always advised me to operate the air con regularly during the winter to keep it functioning properly and they weren't aware there was a minimum operating temperature.