Yep, but (at least in principle) my network is fine and the Pi was connecting to it before.
This may be one of those cases when once I've configured it, it will be fine. But for an update or a new connection, to not boot rather than reverting to an empty database does feel like a regression.
Anyway, I'll do some troubleshooting over the weekend and report back.
Moved it to my desk. Plugged the wired network in. Everything worked fine. Then I went into the settings and saw that it was using wifi not wired. Moved it back to its normal place. Still working.
I do find that live/recorded TV doesn't work well on this one, so it may be that wifi signal isn't great at that place.