I have the same problem (Glow-Worm fuelsaver MkII).
The problem is a hairline crack in the heat exchanger (above the burner). Water seeps out turning any dirt into a carbon-type build-up, which blocks the heat exchanger fins, thus blocking the flue. The gas visible burns with a yellowish flame, which is Carbon Monoxide, although where the boiler is situated has plenty of free-flow air to disperse the build-up.
Once the hest exchanger fins are cleared, all is well, although the build-up process begins again, needing clearing every two-three months.
Peter Squires, January 2011
