The aircraft was being used for dual training and was operating in the circuit, having been airborne for about 45 minutes. During a turn onto the crosswind leg the engine partially lost power. The instructor immediately took control as the power was inadequate to maintian height and initiated a turn for landing on the reciprocal runway. On the approach, the aircraft struck trees bordering the edge of the strip and dived vertically to the ground. The instructor's seat attachment was damaged during the collision with the trees, prior to ground impact. Examination of the engine's spark plugs indicated the front cylinder had ceased to deliver power. The partial loss of power was the result of dirty fuel. Contamination was found in the float bowl of the front cylinder carburettor, in the fuel filter,and in the fuel line between the fuel tank and filter. The contaminated motor vehicle fuel was purchased from a nearby service station on the day prior to the accident. Two stroke oil was mixed with the fuel by the aircraft operator and not adequately filtered before being added to the aircraft fuel tanks.