Put the front of the car up on stands, open the rad cap, start the car, let it get up to temp, get a large pipe and some water, place the pipe in the rad where you would normally put in the water, and then pur in the water down the pipe to get it up over the level the rad can hold and turn the bleed screw slowly. Once all the air has escaped then bring it for a spin. See if this helps.

It's also worth while making sure that the Heater control valve (under the battery tray, at the fire wall is working). Also do you have digital air con or the non digital? If digital I think the motor or resistor can be an issue.

Let us know how you get on.