Lost "All Sent" folder in Mail on MacBook Air

My Mail "All Sent" folder has disappeared from the sidebar and there appears to be no way to add it back.

This issue has been reported by others, but generally the problem appears not to have been fully understood. The All Sent folder, like the All Inboxes, All Drafts etc folders, is a high level directory to the corresponding folders for all of the email accounts active on the pc. You can either view the mails as a single list or expand the directory to see them by individual email account.

The MacBook Air is running Monterey 12.1 and this feature works differently to the implementation on Catalina. Previously the equivalent of the "All" folders appeared under the Mailboxes heading in the sidebar, instead of under the Favourites heading where it now sits. As far as I can recall, these high level directories were always set up automatically when first setting up the system, but having now disappeared, I can find no way to add it back.

Any suggestions gratefully received.