The pilot-under-check was conducting his first circuit in the aircraft as part of a conversion onto the type. On final approach he realised that he had not selected carburettor heat, and moved to apply this control. However, he inadvertently pulled out the mixture control and all engine power was lost. Both pilots tried to return the mixture control to the full rich position, but found that the control could not be moved. The approach was being conducted in strong wind conditions and the instructor assessed that the airfield was beyond safe gliding range. He took control and turned the aircraft towards a cleared area. However, the aircraft overshot this area and collided with trees before coming to rest in a ditch. Examination of the mixture control revealed that the cable had broken due to metal fatigue. However, the cause of this fatigue was not positively determined.