How can I diagnose my Glow worm Fuelsaver F100 boiler problem?

My ageing, but until now perfectly reliable, Glow worm wall mounted central heating boiler appears to have developed a fault and I would like to know if it is possible to repair, or at least diagnose the problem. When I turn on the system the water pump comes on, the boiler exhaust fan turns on, the piezo starter ignites the pilot, the main jets fire up nice and blue. All looks well for about 30 seconds and then the jets go out and there is a couple of clicks and silence. Leave it for a few minutes and the whole boiler lighting sequence repeats. I end up with vaguely warm radiators and warm tap water but nothing more. There appears to be a good forced air flow on the outside vent, and nothing appears blocked. I have replaced the circulating pump thinking that had gone. The three way valve appears to operate ok.