With help from nVidia
The feature update to get past 1809 failed repeatedly on one of my systems - didn't appear to like sharing the EFI partition with GRUB. Eventually I was forced to do a manual reinstall of Windows to get up-to-date because the latest nVidia drivers refused to install on such an "old" version of Windows.
Let's hope that Microsoft have fixed such update issues in this release - not going to hold my breath though.