Experienced climbers in a familiar area on a simple warm up climb, first climb of the day. Topo for climb taken from sportclimb.co.uk as no guide book for DM walls existed at the time. Topo had no route lengths in it. 30m rope was used where majority of route known to be 15m. Climber used belayer rope and kit, unfamilliar and not wanting to scrutinise to much for fear of offending. Climber had a 50m rope in the car with proper belay sheet where rope ends were always tied. Belayer had a 30m rope in an ikea bag. Upon lower off rope was found to be too short, belayer continued to lower off, no knot tied in the end of the rope therefore rope slipped through belayer and belay plate. Climber fell four meters and is now paralysed. Route assumed to be 17m not 15m.