THORN BOILER REFUSING TO HEAT UP WATER / C-HEATING?

My Thorn boiler (model: Olympic 20/35 CF) has developed a problem. It won't fire up to heat the water or central heating. I have to fiddle about with the timing switch and when it does fire up, it does so for about two minutes and fades back to being a pilot light again. Does anyone know what the problem might be? I'm so apprehensive about having to call out a boiler repairer who might tell me I need a new boiler (when I may not) with the amount running into hundreds of pounds). Thanks for any advice.