Gas flame always low

[steq184]

I have a Leisure Bistro 55 cooker, about 10 years old, the 4 hob rings and the grill are working perectly.
My problem is that no matter what gas mark i select the flame stays the same very low flame in the oven.
I have been told it could be the thermostat or the Flame Supervision Device (FSD)
I was under the impression that the FSD actually shut off the gas if the flame goes out.......but as the flame is lit..although very low could this be the problem?
Any help greatly appreciated.