The aircraft departed with a planned fuel endurance of 170 minutes, and an expected flight time of 116 minutes. About 110 minutes after take-off the engine lost power and the pilot was forced to land on unsuitable terrain. The aircraft sustained damage to the wings, landing gear and rear fuselage as it ran through two fences and came to rest after striking several trees. Initial inspection revealed that the spring loaded fuel drain cock for the right tank was in the open position. The aircraft was parked on wet grass when the pilot conducted the fuel drain check and he evidently did not visually check to ensure that the right wing drain cock had resealed. When the engine lost power the pilot switched the electric fuel pump on before changing tanks. This sequence is the reverse of that recommended by the manufacturer and increases the time taken to effectively restore power. Adequate fuel had remained in the other tank to allow completion of the flight, had engine power been regained.