All this info seems pretty good for jerry shaw, but no specifics.
by all mean correct me if i wrong but this is how i have test my air con on my other cars. get the car running and watch you fingers.
1. the canister with the view port on the top of it, at the front left of the engine bay as you stand in front of the car. This canister has a connector on top of it pressure sensor mention earlier in the thread. remove this connector and short it out with a bit of wire (or equivalent anything will do).
2. switch on the air con on fan to max and air con on (snowflake switch to on not snowflake ECO as this is lower power air con).
3.there should be an audible klunk. which is the compressor clutch kicking in.
4. you can disconnect and connect the pressure switch short and see the clutch clicking in and out.
5. this shows it is a low pressure gas fault. but does not guarantee the compressor will compress the gas, only that it will engage and disengage. if the rest of the system is gassed up correctly.
6. one thing to try if the clutch does not klunk over when it should is give it a bit of a whack or gentle tap if you like, sometimes the clutch can stick if it has not been operated for a long time as mine did on my jeep. i gave it a bit of constant pressure with a metal bar and switched it on and off a few times with the pressure switch short method, which started it working but then it stuck again so a bit of wd40 and then some 3 in 1 oil kept it freed up and it work from then on. I know it sounds a bit odd putting oil on a clutch but it worked, the oil goes behind the clutch on the shaft of the compressor.
hope this helps.
Cheers dave