The helicopter had just taken off on a short flight. The engine started to run rough and lose power. As the helicopter was over a large open area, but surrounded by a built-up area, the pilot lowered the collective pitch lever and initiated an autorotation. During the autorotation descent the engine stopped completely. The pilot successfully performed a power-off autorotation and landed the helicopter without damage. The Investigation found that carburettor icing was the probable cause of the event.