Who is your email provider? Is this an IMAP or POP3 account?

It's possible that your local folder is corrupt. You can try Thunderbird's folder repair tool - right-click on the folder in the left-hand pane and choose Properties | General Information tab, and click the Repair Folder button.

Hi - email provide is Google gmail. Not sure if POP3 or IMAP - don't know how to check this. Tried the Folder Repair but that hasn't solved my problem. Old emails are again being restored after deletion (both in Thunderbird Inbox & Deleted folders and then again in Gmail Bin via Firefox). Thanks for the thought but the problem seems to be more difficult to solve. Simon.

You can tell if IMAP or POP3 by looking at (on Windows) <Tools | Account Settings | (email account) | Server Settings>. There you will see the Server Type.

In this case, it might be most expeditious to just go to the Gmail web interface and delete the messages there. Keep in mind that when Gmail "deletes" a message, it goes into the Trash folder and will still be included when you look at Thunderbird's "All Mail" folder until Trash is emptied. This is a quirk of Gmail and is why it's recommended to unsubscribe in Thunderbird from All Mail.

After you delete the messages on Gmail web, you should Compact all the folders on Thunderbird.