Anti-Virus products may be intervening and helping to cause delay. Do you have any Anti-Virus product which may be scanning folders or documents you are uploading to server? If yes, then suggest you make Thunderbird profile folders exempt from scanning.

One way to test if something is causing delay: Exit Programs. REstart Computer in 'Safe Mode with Networking' (note you will probably need to connect computer to router via ethernet cable in order to do this) Start Thunderbird Test moving email from the 'Local Folders' mail account into gmail imap Inbox.

Thanks, it looks very much like the issue wss at Google's end as without making any changes it suddenly started working. I suspect it was a upload limit that I had exceeded for IMAP as I was moving a large number of emails from one email account to another. I had waited 24 hours thinking that it was a daily limit but it was more like 48 hours...