Hi CraigShone,
Thank you for posting in the Microsoft Community Forums.
Your goal is to downgrade the old old domain control and retire the domain is it?
How do you confirm that replication has been confirmed as normal?
The extra "BUILTIN\Administrators" in the folder in the old server cannot be deleted, which is also a type of permission group.
You don't need to worry about this error if you just want to demote the old server. The extra group "BUILTIN\Administrators" exists on the 2012 server.
You can just downgrade and reclaim the old server normally.
Best regards
Neuvi Jiang