Burner flames all over the place?

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.

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