Sounds like the thermocouple (flame failure device)has decided to quit. They are not repairable but replacements are a common item.
Try typing 'Thermocouple for belling cooker' in a Google search bar and there are pages full of them.
Geoff, deep down in Wales
May 2010
Automatic Answer
While you wait for answers, we may have found some information which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix gas cookers.