Thank you for your offer of help, MoonJ!
All aspects of my Touch ID work perfectly (thanks for those inks; am familiar with their content): it's ONLY APPLE PAY which fails to launch when it should.
I have to go to my Wallet.
Previously, resting a clean finger on the Home button invariably launched Apple Pay… I know because I did it so many times by mistake :-)
That has stopped working. Even if I leave a finger there for 20 seconds. All other Touch ID functions work as they should.
No changes to iPhone other than regular approved updates. Restarting makes no difference: I can now only get to Apple Pay by manually navigating to the subfolder in which I keep Wallet. Tap on Wallet and Apple Pay launches as expected.