Hi - I had this problem too. I assume you are referring to the automatic oven, not the manual one. There is a small 'Licon' switch behind the control knob, which provides electrical power to the main gas valve in the oven. It is activated when the knob is turned on and is also connected to the timer clock, so that power is only provided when the timer reaches the cooking start time, or the timer is not being used. I had this replaced by an engineer from New World (at great expense!). Changing it seemed to be fairly straightforward - the engineer removed the two outermost knobs and unscrewed and removed the facia. The switch is behind the main oven knob - but where you might get one from, I don't know. And don't forget to switch off the power before you try to access the switch, as it is likely to be live.
Good luck
Keith E