Taking into account all your comments and hints, I’ve just reworked the image again.
Difficult to tell if zooming helps because I am usually quite zoomed in already to do the retouching.
I didn’t notice that much difference in hiding masks and disabling the palette, at least for what I was doing.
Pascal, I now understand what you meant about using already cloned areas as a source - thank you, it was very useful in my new test.
I will add another comment on using the clone tool, however. Having to hide the masks to see the effect of the cloning, then having to reshow them to make the next correction, is a real nuisance. I feel having a dotted coloured outline would be far better than a filled area - at least whilst editing an existing cloned region. I seem to be struggling for how to describe what would be best - can someone please help argue this out?