1st thing to check is that the AC rad / condenser is clear of any debris and there's nothing in front of the air flow to it, next ensure that the AC pump drive belt isn't slipping and if any doubt replace it.
Also check that if there's a pollen filter to the cabin air inlet that this is clean and free flowingNext you can test components simply by checking the temperatures by touching them with the system running.
with the engine at operating temperature and the AC turned on it should be as follows:
A hot compressor / pump
A hot high pressure refrigerant line from the compressor to the condenserA hot condenser on the inlet
A warm condenser on the outlet
A warm liquid line from the condenser to the evaporator (before the expansion valve or orifice tube)
A cold evaporator outlet line
A cold low pressure refrigerant line to the compressor
If you get a different result from the above then suspect a blockage in the line or the component whose temperature is abnormal, this can be confirmed with a pressure test
I'm off on holiday now so I've left the problem to another Tech, attaching your answers to the job card for him..
He's now got a further problem as what was a fully functioning compressor clutch yesterday is now gone U/S !
I think the clutch is a false alarm, the Low setting is 16.5C and I now realise the ambient was 15C at the time.