During the pre-landing checks the right main and nosegear gave normal green light "gear-down" indications. The left maingear green light remained extinguished and the red gear warning light illuminated. The instructor took over control of the aircraft and proceeded to carry out the emergency procedures, including recycling of the landing gear selector. Selection of "gear-up" resulted in no change to the position indicators, and no audible indication of the landing gear moving up. When power was reduced, the audible warning for "gear not locked" was heard. The emergency extension procedure was then carried out but was also unsuccessful. A number of low passes were carried out under the observation of ground engineers who reported that the left gear appeared to be not fully down. Yawing and "g" manoeuvres failed to lock the gear down. During the subsequent landing roll the left maingear collapsed, the aircraft swung to the left and departed the runway. At this point the right maingear also collapsed. The failure of the normal landing gear extension system was attributed to lack of hydraulic pressure but the reason for this was not determined. The failure of the emergency landing gear system was the result of a binding downlock assembly which did not allow the lock to fully engage. The landing gear balance spring was too weak to pull the left leg overcentre. Subsequent receipt from the manufacturer of new springs of different tension is considered to have solved the problem. This accident was not the subject of an on-scene investigation.