My oven wont heat up?

I have a Belling gas cooker in my caravan, its a full size unit with oven grill and four rings. There is a fault in that the oven will light with a very low flame but the rotary control knob does not control the flame to increase decrease temperature. Can anyone suggest the cause of the problem. There is decent gas pressure as all four rings will burn no problem. Could it be the Thermocouple?
thanks