Last week I got the aircon of my car regassed, hoping that was all it needed (No cold air from the vents even at LO setting). The aircon specialists tried a gas refill, no leaks were detected, but there was no change to the aircon performance, so they reckon the compressor is at fault. There were no fault codes from the car diagnostics and the climate control behaves as normal when choosing different fan and vent settings apart from the lack of cold air.

So before I pay them quite a lot of money for a new or second hand (They're currently looking for a deal on a second hand one to offer me two prices for the work) compressor:

Is there any way of making sure the compressor is at fault in this case?

Given it's likely the clutch to drive the compressor is where the fault lies, rather than the compressor itself, can this be sourced and replaced separately?

Is this the correct part for a 55 reg Octy 1.9 PD and is this a good price? http://cgi.ebay.ie/AIR-CON-PUMP-COMPRESSOR-SKODA-OCTAVIA-VW-CADDY-3-GOLF-5-/330426084712?cmd=ViewItem&pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item4ceeeded68

What is the 'filter dryer' which the ebay page recommends should also be replaced with the compressor and what does it do?

I have an identical issue, did you get anywhere with your repairs??

I bought the new compressor off ebay and got it fitted and the aircon worked for about 3 weeks gradually getting weaker and weaker before stopping working again. Turns out there was a small hole in the evaporator heat exchanger. The leak was detected thanks to the refrigerant having a UV sensitive dye added to it, so all that needs to be done to find a leak is to look over the car with a UV torch. Fluid was spotted under the clutch side footrest leaking down from behind the dashboard and a small amount was seen where the refrigerant pipe goes in through the heat shield from the engine bay, suggesting the leak is in the evaporator behind the dashboard (Not something I'd plan to pull apart without plenty of time on my hands and a very good set of instructions, so a labour nightmare even through the leak could probably be soldered over and pressure tested for £50 once the evaporator is out).

The leak wasn't detected initially because the standard pre-filling test is to pull vacuum on the system to check for leaks (Hold a vacuum for about 30 mins, the system fails if there's any change in the pressure reading), which will detect big leaks but won't detect a tiny hole that's pulled closed by drawing a vacuum on the system. Given the system runs at between 12 and 2 bar (Depending on where in the system you look, 1 bar is about atmospheric pressure), there's plenty of scope for the refrigerant to leak out over time if there's even the tiniest of holes.

When I brought the car back in after the aircon lost pressure the new compressor appeared to be broken too, probably due to moisture getting into the system through the leak or possibly due to the system being contaminated beforehand, so to conclude I decided since I'd already thrown about €400 at the problem and there's no guarantee that paying the same again would fix it anyway, it was a waste of time continuing trying to get it fixed.

From what I can gather this is a relatively small amount to be stung for in trying to fix a car aircon system. I should have just followed the advice I got from one of the other forum users here and get a belt from a non-aircon equipped engine, remove the compressor and run the shorter belt on everything else.

It's worth trying a regas (Make sure the compressor is working first though) as this will be under £100 and might be all that you need. Go over the system with a UV torch before you bring it anywhere though, if it's a leak somewhere inaccessible then it's potential money pit.

...the standard pre-filling test is to pull vacuum on the system to check for leaks (Hold a vacuum for about 30 mins, the system fails if there's any change in the pressure reading), which will detect big leaks but won't detect a tiny hole that's pulled closed by drawing a vacuum on the system...

The correct test is to fill the system with OFN (you shouldn't fill a system with refrigerant if you think it is leaking). As psycholist suggests, vacuum testing a pressure system doesn't produce definitive results.