On returning to Kemeys Commander Airfield in Monmouthshire after a short local flight, the pilot made an approach to grass Runway 13. The runway surface was damp and the wind was from the south-east at 5 kt. The pilot estimated that he touched down between a third and a half of the way down the 500 m runway, bounced once and subsequently overran the end of the runway, over a rut and into a hedge. Both occupants, who were wearing full four-point harnesses, vacated the aircraft without difficulty. The pilot attributed the accident to his unfamiliarity on type coupled with an approach which he believed was flown too fast.