The morning started with the boiler banging away, so I went to focus and bought some fernox central heating restorer. It said drain the system and run for a week. So I proceeded to drain the system I attached a hose to the drain cock and the water began to drain away then it stopped . When I checked the header tank it was still full . I guess I've got an airlock, have tried attaching a hose to the outside tap and filling the system up this way, but no luck. Has anyone got any ideas what to do.