The pilot proposed to conduct a flight around the Station windmills. His attempt to start the aircraft was unsuccessful owing to the battery condition. He turned the ignition and master switches OFF and proceeded to hand swing the propeller. As he turned the propeller, the engine fired. The throttle had been left half opened , the park brake was not engaged and the aircraft was not chocked. The aircraft moved rapidly and the pilot was unable to gain access to the cockpit. The aircraft passed through a fence and subsequently came to rest in a clump of trees. No defect was discovered to explain how the engine could start with the ignition and master switches selected OFF.