During the outbound flight from Clench Common, the pilot felt that the aircraft was unable to maintain altitude and he performed a precautionary landing at a nearby airfield to investigate. Following checks of the airframe and engine, he took the aircraft for a solo circuit, during which it performed normally. He decided to return to the departure airfield with his passenger but, shortly after takeoff, the engine power again appeared to be insufficient to maintain altitude. The pilot decided to force-land in a field, during which the aircraft was badly damaged. No reason was found why the engine might not have been able to deliver full power.