During the take-off roll the aircraft did not achieve flying speed by the point where it was normally expected to become airborne. The pilot abandoned the take-off and applied maximum braking, but was unable to prevent the aircraft over-running the strip. The aircraft broke through the boundary fence and came to rest in a small ditch. The reason the aircraft did not achieve flying speed could not be positively determined. In this aircraft type, it is important that the elevator trim is correctly set for take-off. If nose-down trim is inadvertently applied, excessive rearward pressure is required on the control column in order to raise the nose. However, the trim position set by the pilot was not established. When the take-off attempt was abandoned, there was probably sufficient strip remaining to stop the aircraft. However, the brake linings and discs were worn to the extent where full brake effectiveness could not be obtained.