Belling G755 Cooker oven wont stay lit

I'll give this a go! With a cooker like this, you depress and then turn the control knob, press the ignition button and continue to depress the control knob for a few seconds. This is because a device called a thermocouple is heated by the flame and you have to continue to depress the control knob so that sufficient gas flows to heat up the thermocouple. If the thermocouple does not get hot or stay hot then the gas valve is automatically closed - stopping the gas. The thermocouple is a 'flame failure' safety device. If the gas gets cut off then the flame goes out and the thermocouple goes cold switching off the gas valve automatically. However, you are having to try and manually keep the gas going for a good five minutes. I would think that there is either a problem with the thermocouple or the gas control valve! Tony www.birminghamelectricsltd.com

Does the knob touch the facia plate whilst lighting gas hob? If so pull it back on knob softly 2 to 3mm so when fully pushed in whilst lighting gas hob, knob should not touch the facia plate. Therefore the valve is not fully open so thermocouple is not activated and hob goes off and does not stay lit.
This is a simple fix for newer gas hobs. It might even help by removing knob and dropping a small piece of plastic to pack out the hole so knob cannot be pushed in to far.