Thanks Roy, that's useful. The clearance job is really good, but I think the damage must've come before that. The big slab isn't lying on the side in the earlier picture, or in Audrey Henshall's plan of the site.
This is a very interesting illustration. I was under the impression that Devil's Den had been erroneously restored as a cromlech in the early 1920s - in fact it looks fairly close to how it looked in 1825 so perhaps the 'well-intentioned antiquary' who restored it didn't get it so wrong after all.