In reply to Neil Williams:
A possible alternative - if it's just for top roping, nearly 20 years ago Plas y Brenin's wall just had a sturdy smooth cylindrical metal bar bolted horizontally at the top of the routes. To top rope you could just pass the rope over the bar, and even increase the friction for heavier climbers and lighter belayers with more turns. I don't know if putting more turns in twists the rope over time.
This isn't just yet another of my mad ideas - an instructor there emonstrated it to myself and others on an SPA course.