So I'm looking at the hourly UKMO forecasts for tomorrow (for Ely, East
Cambs) and currently it's showing a 3o% rain probability for each hour
from 1200-2000 inclusive (but by 2100 it's miraculously dropped to 5%).

I assume that this means that in any one of these hours there's a 30%
probability of more than a token amount of rain (though I don't know
what the threshold might be). So in that 9-hour afternoon stretch then
surely that means that - barring statistical flukes and recognising that
each hour is nominally independent - there's something approaching 100%
probability of some rain. Or am I misinterpreting the way the forecast
is presented?