Well, I've finally got heating, the circuit board, gas valve, electrode and thermistors have been changed and it seems to work! No one seems to know what the fault actually was as the heating started to stay on when the timer was at 'on' but when it was at auto, it would go out and the red light came on. It seems to have baffled the two gas engineers who (finally) managed to fix it.
Thanks for your help and advice guys.
Jacqueline, November 2008