The clutch at the front of the compressor is basically a big electrical magnets. When it receive power from the relay it energize and connects the spinning pulley and fan belt with the compressor. You have already observed that the front clutch is not turning so therefore the compressor is not turning and if the compressor is not turning the ac is not working.
There is a wire with a connector for the compressor. That is the wire to energize the clutch. You can remove the wire at the connector and see if the wire is receiving power from the harness. I'm pretty sure that there will be no power at the harness with the ac button in the on position. You will have to locate the location of the relay as I live in the States and the temp right now is -15C and I don't want to be outside fusing with my car right now.
If you can't locate the relay I would suggest that you take the car to a specialist and tell him that the clutch for the compressor is not engaging. he should be able to locate and fix the problem. It could also be a bad pressure switch.