If the oven lights but the flame stays the same and doesnt go bigger then its a thermostat fault im afraid. Ive seen and replaced loads of them for this fault as it is a fault we seen a lot.
To fix this you will need to have the thermostat replaced and that will cure the fault from there for you.
Hope this helps you, If you need anymore info just get back to me.
I see the question is still open. Is there anything else you need to know on this?
If not then if you could remember to rate the answer by clicking the star ratings at the top and then it will close the question.