My Baxi Wentworth Gas Fire won't switch off?

My Baxi Wentworth Gas Fire won't switch off?
The fire ignited as usual and after about 15 mins there was a loud grating sound like a mechanical noise and the electrode ignition started firing away with noise really loud and the flame still lit? When I tried to switch it off at the controls on the side panel it would not switch off. I had to switch the power off at the socket? When I switched the socket back on, the fault sequence restarted and again I had to switch off the power at the socket. What has happened?