Emergency shutoff won't reset on Glow-worm Betacom 30?

The emergency shutoff light on my Glowworm Betacom 30 combi boiler has come on and will not allow me to reset the boiler. The instructions recommend switching the boiler off for a minute by tuning the dial to the off position, but the emergency light stays on, and the burner still fails to ignite. Switching it off at the mains also doesn't work. I can hear it trying to switch on (the click) but nothing happens - no hot water, no heating. The gas supply seems fine (the cooker works) and there is nothing obviously wrong on the external pipe at the side of the house. Help.