After upgrading my iMac from Sierra to High Sierra, I started getting a lot of really annoying popups from Time Machine along the lines of "This mac hasn't been backed up in 416 days".

Yeah, I know. I turned you off you silly thing ...

I HAD been using TimeMachine to backup to my Synology NAS, but ended up turning it off when I moved everything into the most excellent Microsoft OneDrive - so why was it nagging me now?

Turns out there is a bug in High Sierra (yeah, shocking isn't it), it was now complaining at me as the target volume was not available - turns out you have to get it to "forget" the target volume, otherwise it will nag you even though its disabled.

To do this, go into System Preferences, Time Machine, and right-click on the ICON for the drive.

And there you go, peace at last!