Quick follow up. System Restore did complete. But as I feared, the computer is snail-like, and Task Manager shows the disk usage at 100%. CPU running at between 5% and 15%. Hard to tell what specific processes are hogging the resources, as most show 0% apart from svchost and System.
If it is that pesky and persistent updater trying to write to disk, how can I stop or disable it given that (I assume) the 1709 build-update files have been downloaded and the updater is repeatedly trying to install it?