We were about to set off for Spain on Monday, got in the van and switched the AC on... Nothing. I connected my gauges, the compressor was running and back pressure was steady at 30psi, but condenser fans not running. Turned out it had just lost enough gas to stop working. I just kept putting bits of gas in until the condenser fans ran continuously (I rememberd they used to run continuously)
If there's a click from under the bonnet when you press the AC button (with engine running) that's the clutch bringing the compressor in, so it might just need a gas top up, I think halfords sell it.
PS if you're doing it yourself, don't put too much gas in thinking it will work even better. Too much refrigerant in the system is REALLY bad for the compressor.