- click the account in folder pane - rightclick and select 'subscribe' - untick the box for allmail (and also Important if that folder is ticked) The Allmail folder and Important folder are duplicates of your other folders. - after doing so, you can remove the now unneeded disk space by clicking help>troubleshootinginformmation, scroll to 'profile folder' and click 'open folder', and then exiting thunderbird, and then clicking the imapmail folder, followed by clicking the Gmail account folder and then delete the allmail,allmail.msf, important, important.msf, and then restart thunderbird. :)

Thanks. After unsubscribing I shut down my laptop and when I restarted the Allmail files had already disappeared. I had not thought that this would prevent searching Allmail on my laptop but I can still do this on my mobile. I presume everything will go back to as it was before if I subscribe to Allmail again - I am going to do this next. Then I shall have a rethink and maybe instead delete some substantial ImapMail folders that are redundant.

You're welcome. Just a reminder: in searching Allmail, the only unique messages you will find will be the ones that are not in inbox, sent, or any of your other subscribed folders. Accessing Allmail also doubles your time to send and receive mail and increases all resource usage.

As it happens, after restarting my laptop with Allmail subscription renewed, Thunderbird managed to include it again using about 2GB less than before. So problem solved anyway. Thanks