When you understand the process, the LibreOffice mail merge system works quite well.
But when you have your finished merged document, containing all your address labels, it's impossible to edit any of the labels other than the first one. If you try you to make an edit you just get a pop-up message:
Read-only content cannot be changed. No modifications will be accepted.
This is annoying if you spot something that needs a minor edit, and you don't want to go back through the whole process.
The answer is to remove the read-only protection by: