The aircraft touched down normally on the 760 metre long strip and the pilot commenced braking immediately. He was then distracted by the sudden appearance of two cyclists at the edge of the strip and was concerned that they might attempt to cross the strip in front of the aircraft. This did not occur and the pilot, returning his attention to the landing roll, realised that the rate of deceleration was less than expected. Despite pumping the brakes, he was unable to prevent the aircraft from over-running the end of the strip and colliding with a large rock. Heavy rain had fallen on the strip shortly before the aircraft arrived. The grass surface was slippery and resulted in reduced braking effectiveness. When the pilot realised that the rate of deceleration was abnormal there was insufficient strip length remaining to ensure a successful go around.