I *might* have solved the battery issue. I did two things:
  • Removed persist.rildenabled=1 from build.prop - this got added by the autoflash app I used to install the beta and I don't think it's required.
  • Uninstalled LiveWare Manager which I'd installed soon after upgrading to the beta.

About an hour ago I checked Android OS usage in Battery and it had been awake for 6h 59m (more or less the time since the last charge). Now it says 7h 3m - and I have switched the device on once or twice.

Could either of those changes of made a difference?