I found another post
here
which suggested clearing the icon cache: in my Windows 11 this was the suggested command:
It has done the trick for nearly all the icons: I suspect SC is rebuilding something in the background and they will all appear soon. Fingers crossed!

... update ten minutes later. All OK now.

Spot on! Thanks for a quick reply.

1. Usually 'add shadow to image' from the menu. I am only recently, somewhat belatedly after a decade or so, becoming familiar with the toolbar buttons.
2. Usually to keep SC minimised.