Thankfully it looks as if things are not as bad as I feared.

Here is Toby's update I received yesterday....

Just to let you know the result of our visit to Belas Knap. The stones in the chamber had indeed been lifted from the drystone boundary wall and thrown into the chamber. They have been tidied up and the wall will be reconstructed by our maintenance team when they visit to cut the grass in the next couple of weeks.
Again many thanks for bringing this to our attention.
Regards
Toby Catchpole.