@peterjones Thank you!
Appreciate the effort you’ve put in. I tried the portable version and found the key combo still didn’t work so, as you said, that points to an external hijack.
I was planning to start the process of elimination following that failure but, the first time (and only the first) I tried Ctrl-Alt-C in the portable NPP, I noticed a very brief disturbance somewhere else on my display and I recognised the source of it. I use a program called MyPhoneExplorer to manage data on my phone and it looked like a progress dialog from that - lasting well less than a second, I would say. When I checked all the settings for that program, I found it has allocated Ctrl-Alt-C for copying selected text to the phone’s clipboard - I wasn’t even aware of that feature.
Once again, many thanks for helping me find the culprit!