Intermittent fault for past year. HW goes cold and boiler pulses with green light flashing. Just once this was followed by a F1 fault code - ignition failure. We switch boiler off, back on, let it run on central heating only for a while then everything OK again. Happens about once or twice a week. Everything else fine. Is it something to do with the pressure valve. Our"service engineer" had no idea and did not come back - good news is we hadn't paid him. Am I going to have to get Glow Worm out - I hope not!