Actually, there was one other problem I had I totally forgot about.
I run VirtualBox on my Studio PC running a 32-bit Windows XP VM, mainly for some old 16 bit apps (like Korg X5D / NX5R synth editors), and some other old apps I find useful.
VirtualBox requires vt-x to run, which clashes with Microsoft's Hyper-V. I had Hyper-V disabled, but after 1803 it seemed to think it was back, even though the system claimed otherwise. 1803 must have partially installed it again somehow, as the services were all back.
If anyone else has this setup, to fix it you need to fully enable Hyper-V and disable it again:
Run powershell as administrator, and type:
Enable-WindowsOptionalFeature -Online -FeatureName Microsoft-Hyper-V -All Let your machine restart etc.
Once you've rebooted, run powershell as administrator again and type:
Disable-WindowsOptionalFeature -Online -FeatureName Microsoft-Hyper-V-All M.