I have a potterton kingfisher mk2 rs80 when it fires up it only stays on for a couple of minutes before cutting out. would this be a thermostat problem
the boiler could be over heating, this a boiler that age i'd bet there isn't enough circulation. you could try changing the pump. But i suspect that there is sludge in the system.
Also this boiler contains asbestos. be-careful if stripping down the boiler
