The eight batteries for the APC 3000 arrived today, so I did the swap-out. I took down the servers that were on that UPS, opened it up and the old batteries came out very easily - so often, I have to lever them out with a crowbar and mallet! The replacements slid in, and the UPS was back on line a few minutes later.
Then I tested the batteries I'd taken out. Six of them looked good, showing 13.5 volts, but two of them were showing 11 and 9 volts, and that's perhaps why I was getting a battery alarm. But six good batteries - if only I could think of a use for them ....
And then it occurred to me; the Mustek 2000s that I have, use the same sort of batteries, and I have three of them out of action. So I pulled the batteries apart (they were stuck together with sticky-back plastic), and one by one, I took the Musteks apart and put in the replacement batteries. I reassembled them, and tested each one by plugging in my soldering iron as a load, and they seem OK.
So now all my five APC3000s are working, plus I have three extra Mustek 2000s to play with. A good day's work.