I have a Halstead combi boiler which has developed a will of its own. Sometimes it will start - sometimes it won\\\'t. Sometimes it will start and then shut off. All this is accompanied by noise which sounds like air bubbles in the system. I have bled all radiators, some of which I can remove the bleed valve completely and no air or water will come out (mainly upstairs) and I have found no air in the pump or automatic air vent and manual air vent on the boiler. This is as far as my very limited knowledge of CH systems will take me. Can anyone help.