Please, please help!!?

Glow-worm Spacesaver 40B Mk11

The burner ignites with a strong blue flame, but after a few minutes, the flames blow around all over the place (as if the pressure has dropped/air in system)...even around the sides of the boiler. The flames then go out, with a few seconds of gas only, and then the boiler (including the pilot) shut down. It will, however, run a little longer when the Combustion Chamber Shield is removed.

Burner pressure setting OK at between 10 bar and 16.3 bar.

The flue is clear...both outlet and intake. Apparently it is 'vitiating'.