Parkinson cowan sonata 55x gas cooker

Question

hi there! i have one of the above ovens but lately the main oven is giving me a headache! The problem is that when the main oven is ignited the flame burns at a constant high setting whether or not i have the oven set for a low heat. This then means i cannot cook something slowly as the flame is always scorching the food. Can someone please suggest what is wrong and which part i need to sort this? May thanksJase