I would guess. copy in the full, then each of the incremental ones in the order they were made, newest last.

As each new mail adds to a single file that represents the folder. That should place the latest mail in the profile replacing each older version of the file.

Just be sure Thunderbird is not running when you change things in the profile (but the same can be aid about making backups. They tend to be incomplete if done with he program running. usually the inbox file is help open for the entire session, so it does not backup unless Thunderbird is closed for instance

Hi Matt, many thanks for your input, it is greatly appreciated.

I have tried exactly what you described but when I start Thunderbird, it shows the same like only the full back up would have been copied in. I tried this various ways but it seems there is a file somewhere in the full backup which is not present in the incremental backups and that doesn't allow the display of the newer messages.

Going the other way around, the last incremental version shows inbox and sent emails correctly but the folders are missing. I even tried to copy the full backup into the last incremental but that hasn't done seemingly anything either.

Suggest copy in the full backup to get all folders.

re:last incremental version shows inbox and sent emails correctly Then only copy in the mbox files eg: 'Inbox' and 'Sent' overwriting the ones that came across in the original first full backup.

mbox files have no extension and contain emails. see image below as guide.

Toad-Hall, thank you, it worked that way!