Unfortunately I said bingo too soon!
I tested a few albums updating the library from my file tags on the NAS, however upon a reboot of the IdPi they did not stick - I.e. The bit depth field was empty.
I still wonder why I'm having to update tags - why aren't they saved when the library is loaded? At least the tag fields with a value.
How are you using the files on your NAS?
As a network share or are you loading it as a DLNA library?
If you load it as a DLNA library, MC's updating of info from the tags will be transient, the MC library in this case is re-created dynamically each time the DLNA library from the server is loaded.
If you want the library to be "permanent" you should mount the NAS filesystem and import the files into MC using the import function in MC.