Thanks, Bjørn and Akira, for your concern - I'm back in again in my normal guise. My iMac now seems to be running very well; I've installed all my most used software; I've hooked up and examined all my main data back-up drives. I'm touching wood, but it does look as if the worst is over.
The next challenge is to get my Mac Pro running again via a "Restoration" start-up from a Time Machine clone. I've never done this before.