I appear to have solved the problem, but I've no idea how I did it!
What happened was I managed with a slip of the stylus to unpair the watch from my phone. I found I couldn't reconnect to the watch without resetting it, so this I did. I then tried to set it up as near as possible to the previous configuration and charged it up. That was 39 hours ago and the watch is still showing 33% available! That's pretty much what I used to get from my old watch, a Galaxy Watch. The only setting that I know I changed was to set the display brightness to Auto.
As I say, I don't know what happened, but it may be worthwhile disconnecting and resetting - you never know!