My Belling dual fuel Country Chef will not turn off?

One of the ovens in my Belling dual fuel Country Chef will not turn off. The knob turns and I can get the temperature low but not off completely (without turning off at the wall). I have no idea which way is 'up' now as I have taken the knob off and it just goes round and round. All the other electric and gas bits work fine. I haven't got a wiring diagram but suspect there is a rheostat or similar behind the control knob (or indeed a replaceable unit). Any ideas?