Paul - even if you consider yourself an amateur, don't hand the car over to a mechanic until you've a better idea of the issue IMHO "It must be the compressor" is the first refuge of the "haven't got a clue" tyre-fitter, and has been used on me more than once.
1. Do you have the fill cert from Kwik fit - how much refrigerant did they remove, and how much did they replace? and the same for the oil...
2. Belt (must be there !!)
3. AC clutch - take the car on a short run, park it up, lift the bonnet and hit Lo with AC on (ac button and light off). You should be able to see (and hear) the ac pulley lock up, and the a/c clutch plate spin. If it doesn't get a spanner (ferrous) and put it near the clutch plate (Warning - these are moving parts, but i assume you are an adult) . If the plate is magnetic then you have power to the clutch (hooray). Whack the clutch plate with the spanner, and it will probably lock on, and cabin will get cold.... this means you need to play with the clutch gap. Use filter paper to clean any crap from between the plates, and if that doesn't work you will need to remove the clutch plate and see if you have a shim you can remove. If the clutch didn't magnetise, then you need to check for 12v and earth at the incoming circular plug on the compressor - if the voltage is not there, or was too low then you need to trace further back through the electrical and control circuit.
4. check fuses/ relays
5. using some extension wires apply 12v and earth to the compressor (i took them off a relay in the fuse box) - does the clutch engage (if so then your compressor, or at least the clutch is most likely fine)?
6. Get a UV light in the engine bay after dark and see if you can see any ( green) leaks especially around the condenser or the lines.
To get 12v at the compressor there is a sequence of events that must hold true - cd be different for a diesel, but for a V8, HEVAC requests AC (Lo on the controller), ECM grants the request, pressure is not too high or too low (pressure switches) etc. If you have a nanocom you can intercept the AC request and grant, to try and identify the "blockage". The trinary switch is probably a bumper-off job, but can be done without refilling the system, not a terribly hard job though.
At least if you pass it to a mechanic at this point you have eliminated the easy fixes and are better armed not to take their BS. But hopefully you are half-pregnant at this point and see it through to the end with the help of the forum : o)