For some reason, this month Windows has ignored my metered connection settings and has downloaded monthly updates without waiting for my prompt. Some of my devices were sat awaiting restart this morning, and some have restarted themselves overnight.
Same again this month. It seems the 'metered connection' hack no longer works to prevent your PC from restarting without authorisation.
I've discovered there is an option buried away under Windows Update > Advanced Options > Delivery Optimisation that allows update downloads to be shared around from other devices on your network. Perhaps this is how Windows is getting around the metered connection block? I've disabled this option on all my PCs. Will see what happens next month...