The aircraft was being flown on an instructor standardisation flight which included the demonstration of a precautionary landing on a grass airstrip. Prior to landing, the pilot in command who was also the chief flying instructor, carried out an inspection of the strip from a height of 500 feet above ground level (AGL). During the first run he noticed what appeared to be a series of ruts along the strip and decided carry out a second run at a lower height. Before descending to 50 feet AGL, the pilot lowered the landing gear as a safety precaution. During the second run, he confirmed that the surface appeared suitable for a landing. The landing gear was retracted and the aircraft was climbed back to 500 feet AGL for the landing circuit. The pilot reported that he carried out the before landing checks on downwind but for reasons unknown, neglected to carry out a final approach check. When the throttle was closed during the flare, the landing gear warning horn sounded. The pilot applied full power to go round but then closed the throttle almost immediately when he felt the propeller strike the ground. The aircraft settled onto the strip and slid to a halt with the landing gear retracted. Although the pilot in command had failed to lower the landing gear this went unnoticed by two other instructors who occupied the front right and rear right seats. It was reported that sunlight shining onto the landing gear indicator lights might have created the impression that the green lights were illuminated.