Glow worm Fuelsaver F swiches itself off?

Usual story ! Having had the heating off all summer, I turned it on for the first time yesterday, everything worked fine then after about 15 minutes the heating went off. Looking at the boiler (wall mounted G/worm Fuelsaver F) the ignition light and burner were out. After about 2 minutes of switching the boiler thermostat on to 5 and off, the fan kicked in again and the heating/boiler were up and running again, unfortunately 15 mins later same again, and so on. Anyone any ideas?