I think Willis' drawing tells a tale Paul. When I first saw the quoit I immediately thought the lid originally fitted directly on top of the sloping flankers. I then saw Borlase's drawing showing that ridiculously thin topped rear support propping it up so had to take a step back and consider. That's where I left it. But look at the capstone, it appears to be purposely held in place from sliding down by a shaped stone which it locks into near the ground!