When preparing for the return leg of a charter flight, the pilot discovered that the engine starter motor would not engage. He elected to hand-start the engine, and briefed the passenger on the operation of the controls. No wheel chocks were employed. When the engine started, the aircraft commenced to move and the passenger's efforts to control the aircraft were ineffective. After travelling about 20 metres the aircraft ran into a ditch and the engine stopped after the propeller struck the ground.
