I forgot to mention, I did check the logs and there was nothing relevant in there.
I disabled one plug-in that was only in the staging server on the off-chance it was causing the problem but after a rebuild the problem remained. I’ve got the same plug-ins on the production server with no issues.
I’ll try safe mode and see if that gives any clues. Other than that I might just manually restore a backup from the production server and see what that does. Last recourse will be a full reinstall.