You might consider an Inverter - takes in 12V DC and pushes out 240V AC. You will need to match the AC Wattage of the cooker with the inverter - it will be detailed on the cooker's rating plate.
To keep the flame-failure alive you will probably need to have the inverter running for as long as you cook.
It may be possible to have a specialist examine the circuit in the cooker - who knows - it may actually run on 12V DC - and wire-in your 12V supply without the inverter.
Screwfix in the UK sell inverters - as does Amazon. £25 should do it but match the Wattage.
An engineer messing around for you will cost much more than an inverter.
Good Luck...
March 2016
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