well by the sounds of it you have done all you can, but i had the same problem and was panicking because i didnt want it too overheat and H gasket to go. We opened the bleeder cork in the front of the engine while the engine was running, get a jug ready to catch the water, this removed the airlock, do you hear gurgling noises? where the radiator air vent is?
i hope that helps Lewey, i cnat suggest anything else im afraid. I give the big fat water pipe which runs across the engine a squeeze few times, and u can hear the water slosh i the filler tank. You cant be getting much of an airlock now, if the heating works?
regards
Chris