I've got a Potterton Kingfisher MF RS 60 boiler which is at least 14 years old. The main burner fires up when I turn the external timer on and then after about ten minutes - 20 minutes goes off but the 'flame' light remains on, and the pilot light remains lit. The fan is working and the external pump is working. The boiler creates some heat in the radiator system before the main burner goes off. Can anyone advise me what the solution is please?