Thanks for the quick response!
They said they weren't in the note, or even the app, so I'm not sure why it would do this if I'd edited it, came out of the app then went back in and the updates aren't there. Sometimes if one of us edits the note and then the other goes to view it, there is a message saying something like 'There's a conflict/newer note available'. Which ends up in loads of versions of the note.