Because of another Zoom session today, I didn't get as much work done outside as I would have liked to. I did, however, move the ladder even closer to the unsafe gable wall of the first of the two stone sheds and continued cutting away some of the overgrowth...
And sure enough, a number of the stones on top of that wall collapsed as I was carefully cutting away the vines and ivy that were all that was really holding them in place.
I knew I was safe, however, as I had the ladder up against the main boundary wall, and the way the stones were hanging meant they would fall away from me, and into the old shed.
Here's a photo showing the current state of that gable. If you look carefully you can see I have since moved some of the fallen stones to just in front of the pile of cut branches outside the shed. Tomorrow (weather permitting) I intend moving the ladder to the other side of the boundary wall, and continuing clearing the overgrowth from that side. I think that will be the safer way to work at this stage...