Check that the a/c pump is engaging under the bonnet - you will see and hear the pump engaging/disengaging if you set it to auto with the temp at it's coolest.
If there's no refrigerant in the system, the pump will not engage.
If the above works, have a look at the dryer (cylinder shaped thing in front of the radiator), on top you will see a small circular window. While the a/c is working, you should see a constant mist inside... if the refrigerant is low, then the mist will be weak and intermittent - time for a re-gas :winky:
Hope this helps :)