So nearly a year later, battery has been fine, but is showing "surface charge" issues again.
So this time, I drained it safely, first with the 3Amp garage lights, then with then 200mA USB outlet/voltmeter on the caravan electrical panel I use for distribution. The low voltage cut out each time cutting it out at 11V. It did not recover during the day, because when it rose above 12V the load got switched back on.
Confirming I had drained it enough, I let it settle for the evening and the voltage read 11.3V, turned the lights on and the low voltage cut in immedaitely.
Then... a "slap in the face" charge. 10 Amps@16V until the CC limiter started dropping voltage down to 15V, when I reset the CV voltage to 15V and left it overnight.
That should fizz it up a bit.
14 hours later it was happily sitting at 14.40V taking about 0.8A, <0.01C.
Put it back into service and we'll see if I can get another year out of it.