My time machine is stuck "cleaning up".

My time machine is stuck "cleaning up". It's been stuck there for about 2 hours & before that it came up with an error saying it couldn't backup (I've forgotten the exact message).

I tried "repair" but it didn't find an error.

It seemed to then backup, but got stuck near the end.

I've just recently updated to MacOS High Sierra 10.13.1 on my MacBook Pro

I've received a system update, which I'll run once the backup finally finishes.