But thanks for pointing that out as that probably was the problem.
[22:18:24] [Server thread/INFO]: Gamerule doWeatherCycle is currently set to: false
gamerule doWeatherCycle true
[22:18:30] [Server thread/INFO]: Gamerule doWeatherCycle is now set to: true
Until recently, I've only ever played that world in singleplayer.
I have no idea how that was set to false.
I shall play around for a while and see if the weather changes.