Well, I think I’ve found the cause.
I ran the battery calibration again, but this time I didn’t let the uCosnole run idle until it shut down; instead, I ran it under load. I played a Full HD video several times using VLC and also set the screen brightness to around 50%.
Whilst the uConsole was running, the battery indicator steadily dropped, and the last figure I noticed was just over 20%. That seems reasonable to me. I then fully recharged the batteries, and now the battery indicator appears to be dropping at a ‘reasonable’ rate and no longer stays at 100% for hours, before dropping dramatically.