Had just finished leading a route at Brimham and found a couple of good belays. Put each of my ropes one to each belay and moved to the edge ready to tie the return rope from my belay back into my harness crab. As I got back to my final position a young non climbing couple appeared to my right jumping across rocks and generally messing about in a precarious position close to a drop. He was pretty competent moving around , she was very uncomfortable and looked nervous at what he was trying to get her to do. The consequences of a slip where they were would have resulted in serious injury. Within a few minutes they had got themselves back to safety and I continued to take the rope in and started belaying my partner up. I looked down when he had started climbing and suddenly thought my knots into the harness didn't look correct. I then realised I hadn't completed my tying off into the belay. I quickly did an overhand knot and tied in. Fortunat;ley he ascended without incident. Had he fallen off in the first few feet before I realised he would probably have been OK. I on the other hand could easily have been pulled over the edge to my death. First time in 50 years I've had an incident like this I'm normally maticulous at double treble checking.