Its probably the Flame failure Device (FFD) this unit allows the main valve to open only when the thermocouple Phial heats up. The phial may have come away from the locating clip and is not directly in the low heat flame,or it is faulty. If you need to change this unit you can access it by taking the bottom of the cooker off. You can get a spare from www.yourspares.co.uk
Anthony Bowen, July 2008