It was working but it stopped and hasn't worked since. There is no supply feed to the compressor when the system is switched on. If a 12V feed is supplied to the compressor the system then works perfectly well and gets cold very quickly.

The control unit is located behind the front bumper on the N/S. There are three permanent live fused feeds plus one ignition fused feed to the control unit which are present and correct. The control unit has a good earth. There is a 12V feed from the A/C switch at the control unit. The fans will work on both slow and fast speeds if the terminals at the control unit plug are bridged. The ambient temperature sensor measures 1500 ohms taken from the control unit plug, and reduces if I blow on the sensor thus suggesting it works properly. The high pressure sensor is neither grounded or live suggesting it's working. Placing a direct 12V feed at the cooling fan switch in the radiator will switch on the relay and operate the cooling fans. Checking the communication from the engine control unit isn't so easy. Despite all this the output from the control unit to the compressor does not becove live when the A/C is switched on from inside the car. Apparantly the control unit has been replaced so I have been told, so if anyone has any ideas I'd be grateful.....