rangemaster 110 oven wiil light but not goto full flame?

when i light the left hand oven it will light but not go any higher then that,i have been told by one person that i will need a ffsd,flame faliure safety device.and been told by another person that if the oven lights it NOT THE FFSD, as if that had gone then it cuts of the gas alltogether,can someone PLEASE tell me who to BELEAVE and if they can help me in wakefield