open your profile folder (Help menu > troubleshoot information and select the open button beside the profile folder entry. Close Thunderbird. Open the mail folder shown in the windows files explorer. Delete the "smart maiboxes" folder in the Mail folder. Restart Thunderbird. That will reset all the automatic stuff that makes up the unified inbox and other unified folders. Hopefully in doing so it will fix itself as it regenerates.

Thanks Matt, When I try to delete the Smart folder, I get a message saying: (para)- "can't do that, cos the folder, or a file in it, is in use". I have no idea what/which folder/file it means. Obviously the Thunderbird is open, cos i'm using it's troubleshoot area. Sorry to sound dim (or more likely being dim), but have you any idea please ? all the best