Not been up there for some time but it may not be just a straight case of vandalism. We have had 2 severe winters which may well have blown what was left of the lime mortar, coupled with the possibility that livestock (cattle or sheep) may well have used the corner for a good old scratch, then there is the chance this may be the cause, just a thought.
We've had a couple of such examples here in the quarry where I live.
It must be 2 years now since the NT took over the Engine House, what works are they doing Lee?
Pete.