If I start the car regardless of whether the engine is warm or cold, the aircon works fine, ice cold as it should, after ten minutes of driving around it goes warm, and nothing you can do with it will make it go cool again unless you switch it off, leave it for 5/10 minutes and put it back on it will be ice cold again

Cant understand why it does this? Theres plenty of gas in it the aircon fan is working you can hear it click in and a rise/drop in revs when its turned on and off etc?