Thanks Cyn, it was fascinating reading. I had no idea I put that much into this topic, earlier.
On a hunch, I went to a spare room and looked at my previous Mac which is stored there - not thrown away yet. The keyboard was grubby and I vaguely recall at least one key doesn't work, but anyway gave it a shot. (It's a Lindy USB "made for Macs".)
Gave Lindy a thorough clean and connected it to an iMac, without much hope. iMac identified it and then, away we go! Tested all keys, which work apart from broken power on/off, so I've just saved myself x$/£/€ in sourcing replacement clicky-clack k/b for now. Great!
On this topic of not throwing away old kit, I'll start a new topic, and hope people see it. I'd like other peoples' thoughts too.