Thank you very much for your help.
I have discovered that the painted foliage is the main problem. While nothing stood out as being wrong particularly, after removing it all, saving the level and restarting the editor, landscape was once again saveable.
It seems possible to me that editing landscape with foliage instanced upon it (even in a different place) might throw an error so from now on I’ll forego the urge to start painting foliage until I’m certain the landscape is finished.