Nigel, you asked the other day if I had any photos proving movement at Trethevy. Have a look at this and form your own opinion.
The first pic is a plan of the quoit by Barnatt in 1982. Look at stone four which has a large gap between it and the stone next to it. Now look at the second pic and see how the capstone has drawn it forward until leaning against that same stone. Stone 4 is the one on the right taken from inside the tomb. I'll leave it to you to decide.