Home heating systems?

Further fault Glow-Worm F100 gas boiler.
When the boiler starts up the burner flame comes through the pilot light port. I thought that this must be because the fan was not working, i.e. not drawing the flame up through the water tubs. I checked the fan and it is running. I am now wondering if the fan is faulty? Or is there another problem Nothing in the manufacturer's literature on this fault