Thanks for your help. Unfortunately it didn’t solve my problem. Many of the settings it suggests changing are locked out due to the service being faulted!
Since I did this out-of-hours, I was able to restore backups for the domain controllers and then load a previous configuration for the TrueNAS box. The restore meant that the NAS is still part of Active Directory (I take backups of both DCs every 6 hours because I’m paranoid, so the backup was literally a couple of hours old). It’s taught me that backups are my saviour and I now need to find a way of automating the TrueNAS config backups - even if that’s just a repeating appointment in Outlook 
TLDR: You can’t (currently) manually remove Active Directory from a trueNAS box without resetting the configuration. In my case I was able to restore from a backup from when it was working and then I have the option to leave the domain.