I guess I'll answer my own question so others that experience this issue know what to do. Something in the folder in question apparently has become corrupted. Perhaps someone more knowledgeable than me can shed some light on this. I've cured the problem
by deleting all the messages in the folder which exhibited the incorrect display and then imported them from my backup file. Everything is now back to normal.
Just a reminder, Windows Live Mail will only import from a desktop folder called "Mail" which you have to create with the necessary folders in it.