Possibly two issues here, check the fan is working correctly and not shot, with this done you can move to the system.

Do you use the air con for at least an 30 minutes per week? if not then the shafts can settle and let the system leak slowly over time, and the refrigerant gas naturally deteriorates. Systems need replacement gas and approved lubricant every few years to maintain their effectiveness, and you really need the system checking. Try Quik Fit as they drain the system, pressure test it, then recharge it with lubricant and gas for a reasonable £50.

If they find a leak during the leak testing, or any other fault which means they cannot recharge it they charge you nothing, so at least you get a free system diagnosis even if you have a leak.