I found information on a very similar error message in here:
This is the relevant information that should apply to your situation:
These messages occur if permissions are incorrect on your designated music folder or on a folder inside your designated music folder. Permissions are settings that determine who can read, write to, or execute a file or folder on your computer. Every file and folder on your hard disk has an associated set of permissions.
Example: If permissions are correct on your Music folder, but incorrect on the U2 folder inside your Music folder, you would be able to add other music to your iTunes library, but not that new U2 album.
Look here for help in correcting the permissions on folders:
Thanks for your message Delgadoh.
I checked the read/writer permissions on the iTunes folder (and applied that to the sub folders) before posting.
It did not help the issue.
To reiterate, this problem has only just started, I've been downloading and syncing podcasts for years without this issue and concerned that it's kicked in now - right after upgrading to the new version of iTunes.
Any other ideas?
Right, I've answered my own question.
I'll put this down to user error with some help from iTunes.
Seems that somewhere along the line, my iTunes has pointed itself to my daughter's music library so I didn't have privileges to write to that user's folder.
So, I've pointed iTunes back to my own library in Preferences > Advanced and all seems to be fixed.