Glow worm Fuelsaver F Boiler cutting out?

I have a Glow worm Fuelsaver F boiler which heats my water/immersion tank. This morning the boiler was firing up and the pilot light igniting as normal but was only running for ~20secs before cutting out again, it would then re-fire again but not stay on long enough and after 40mins, the water was barely warm. Usually I hear the water going through the pipes whilst it's on but this wasn't happening either. Any ideas what the problem might be? Many Thanks!