A volume of air bubbles around the gas-fired (Potterton Profile 50e with
individual stats on each radiator) and it won't bleed out of the rads or the escape value at the tank. 2 plumbers have suggested I fire boiler on max temp. turn on one rad only and bleed it but has made no difference. Header tank is fine (water is rusty coloured but we are in a sandstone area which may cause that) so is there anything I can do before draining and flushing the whole system? Plumbers say this can take many hours which they have to charge for just to watch and wait. Fair enough but I would like to leave this to the last resort if possible.