As it turns out... this corporation.
It did change back after I commented yesterday, but I think I've now got my preference - check for updates, but let me decide whether to install them - to stick.
The problem was everybody's favourite update: KB3038853, the Win10 nagware 'GWX' update.
I originally let that one install before knowing the data slurping plans, forced updates, etc., with Windows 10 - on the basis that I might at some point have opted to upgrade. (Which is obviously no longer the case).
It appears that GWX was periodically resetting my Windows Update settings - which is behaviour I don't think I've seen mentioned anywhere before (or at least if it's been mentioned somewhere I read, I must have missed it).
I killed it and buried its corpse from my system yesterday, and that seems to have done the trick.
Tut fucking tut, Microsoft.