Climber was abseiling to the ledge at the base of subluminal on a doubled up rope. No knots were tied in the end of the rope and when the climber reached the ledge they leaned back on the ab rope to inspect the cliff. The rope was too short and one end slipped went through the belay plate resulting in the climber falling off the edge of the ledge and falling ~30ft down a narrow gulley in to the sea. Fortunately the climber avoided hitting anything during the fall and was uninjured , they managed to swim over to a safe exit from the water.