OK. I assume you have it set to automatic, and have set a fairly low temperature for the car interior, (like about 18C degrees). Is this the case ?

First check the evaporator exit pipe, (the evaporator is the bit within the heater/cooler box inside the car that actually cools the air). This is the larger of the two air-con circuit tubes and goes to the compressor. This should be getting cold. If it isn't then the evaporator is not getting cold because it has no coolant supply. Check the compressor is rotating; this means the centre of the compressor pulley. If it is not rotating the electric clutch is not working either because it has failed, or there is no supply to it. This will be via a relay with a fuse - check the fuse !

Other thing could be failure of the temperature sensors for external and internal temperatures. The one on the inside resides behind a small grill, and has a fan to blow air over it. This can get blocked with crud.

 

Need to put some gauges on it and let us know what the readings are. Also, you are in the wrong forum. You should be in the XJ40 area. No big deal, but you will probably get better responses there.