Like for Bryan, there was a bit of luck involved.
Macrium Reflect was unexpectedly popping-up progress windows - and it finally dawned on me that this was occurring when I was trying to get [Ctrl+Alt+M] working in PL … I checked the Macrium settings and found this shortcut in its settings.
Fortunately, Macrium allows its shortcuts to be customised - so, I could change it to allow PL to receive the key-strokes.
John M