The pilot was carrying out various manoeuvres in the training area. After about an hour general flying, the pilot decided to conduct a practice forced landing approach to a disused strip on the island. At about 200 feet on final approach, the pilot moved the carburettor heat control to the cold position and applied full power to overshoot. The engine failed to respond normally, and produced only about 1500 rpm. The pilot exercised the throttle control without obtaining any further power increase, and he was then committed to a forced landing. Touchdown occurred in a cleared paddock and damage to the nose gear and propeller was sustained when the aircraft ran through a ditch. No fault was subsequently found with the engine or its associated systems. Reference to the appropriate chart indicated that atmospheric conditions were conducive to the formation of moderate to severe carburettor icing. Although carburettor heat was selected at the start of the forced landing practice, it was likely that some icing had already formed and was not dispersed by the time the throttle was re-opened. When the engine failed to deliver full power, the pilot had not re-applied carburettor heat in an effort to restore normal engine operation.