I've established that the compressor is cooling air when I bridge the terminals to the relay. I need to bridge the relay and turn off the aircon (neither light showing) to get chilled air, however, as I can't then close the external air inlet flap, I can't get it really cold. My mistake at the weekend was to assume that the 'recirc' button on the middle dial only opened and closed the flap, as is usual, but it seems that it controls air distribution too based on internal temperature, overriding the right dial. I reckon the system 'thinks' it is colder in the car than it really is, so it's not actuating the relay and is directing the air down into the footwells. It may even be warming the air. The system doesn't seem to actuate the cooling fans either.
It's looking more and more like my problem lies somewhere behind the dash - either within the ECU or with the internal temperature sender. Does anyone know how to check the sender? The outside temperature guage is reading about right. If I get to the ECU, how can I check whether it is faulty?
I'd still like a look at the electrical plug on the pressure switch if anyone can tell me how to release it from below the car ('98 V6).