Thank you all so much for your extremely kind comments.
To answer Seapy's concerns with the difficulties of masking awkward shapes and complex skylines:
The trick is to make a marching-ants Selection of either the wanted, or the unwanted, parts of the image (whichever is easier, because you can easily invert your selection if required).
Once you have an active Selection, one click on the Masking button will add your Selection as a Layer Mask to any Adjustment Layer.
