Thanks for the replies and advice. The voltages were all with the battery off the bike.
I put it on the bike again, and of course, it fired up immediately. I went for a long ride.
After 10 days not riding, again, it started instantly - just in the garage, didn't ride it (bloody rain again!). Then 4 days later I tried again and it tried to turn over and started clicking. Same at each attempt.
Terminals and connections were all tight. At this point I decided it was time to get a new one.
The problem is, this is my second battery in the 3 years I've had the bike. OK, so the first one was in the bike when I bought it - and it had been quite difficult to start from the off, but I rode the bike through winter, too. I reckon the battery it came with was not up to the job. I just got rid of it about a month ago - shame, as I hadn't checked the spec before chucking it. My concern is that the next battery will go the same way.
I blame my wife for wanting to get a car, and then, despite promises to the contrary, refusing to get on it after we got the car
Actually, I blame the idiot bimbo who knocked us off it and knocked Mrs JD's confidence with it to make her want 4 wheels.