Went to start her this this morning and it was real chug-chug, although she did eventually start. But as I have seen before, and now see all the time with my new voltmeter, the voltage, although nominally OK off-load at 12.3V, dropped off to 9.3V during cranking. A sure sign the battery was really failing. So swapped in the new one that my son had got me. Fitted perfectly, although I had to incorporate some adapters to convert the battery pillar terminals to flat bolt-through types. Also added a cut-off valve on the negative terminal that I happened to have lying around.
Result ? Instant start the moment I turned the key. Didn't even
crank long enough to see the voltage drop. Good job done.