I looked under View and also Account Settings but didn't find anything I could do (or knew to do.)
I was investigating the other option (reinstalling the IMAP account) but while I was looking at my missing folders which are still visible on the server, I had an idea - I created a new folder in Outlook (to replace 1 of the missing ones) and then on the server (webmail) moved an email from the missing one to the new folder. The email is there - in the new folder in Outlook. I have 2 missing folders with 7 sub-folders under 1 of them so not too many to move - I can just check the boxes and move all emails at once for each folder. If the missing folders ever magically turn up again, I'll delete them (I've slightly changed the name of the new folder) or I can delete them on the server.
Not a text book solution but I guess if it works . . . unless it's something that will recur because I haven't solved the real problem?