The Pilot-under-Instruction selected the left fuel tank during the pre-landing checks as the aircraft joined the circuit on left base leg. Approximately 20 seconds later, and after the aircraft had commenced descent for landing, the engine stopped. The selector lever was returned to the right hand tank position but the fuel boost pump was not turned on. The engine did not restart immediately and the Pilot-in-Command elected to concentrate his efforts on a forced landing. The aircraft touched down in an uncleared bush area and overturned when the nosegear collapsed. The subsequent investigation determined that the fuel selector panel had been fitted with a decal which indicated an incorrect position for the left tank. The decal had been modified some time prior to the accident, with a coloured pen, to show the correct position. The colouring had subsequently worn off, resulting in both correct and incorrect positions being visible. The fuel selector was also fitted with detents to help locate the correct selection positions. A build up of grease and wear on the detent combined to make the left tank detent less positive than normal. When the pilot had moved the selector, he had not noticed the detent, and had inadvertently shut off the fuel to the engine. During his pre-flight inspection, the Pilot-under-Instruction had noted that the selector was different from that discussed during a briefing given by the Pilot-in-Command on the aircraft systems. However, he had not alerted the Pilot-in-Command to the anomaly.