Ok Tiggs. From what I've read in the manual, when the on/off switch is set to off, the gas supply and electrical supply are shut off.
Either the gas valve isn't responding properly, or there's a fault on the circuit board so that it's not carrying out the shutting down procedure. Either way, you'll need a boiler man to check it over.
It could be something as simple as a sensor gone wrong. A gas valve wouldn't be too expensive, but a circuit board would cost £200 or more to replace. If you can get someone you can trust, he'll give an opinion as to whether it's worth repairing (just looking at the worst outcome.)