Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Hi Matt, many thanks for the quick response. I tried your suggestions but the problem persists. When I try to write a new message the new message window appears but in the body of the message, where I should be writing, it is filled with a thread of 3 previous messages. I have deleted these messages and emptied the trash folder. Still, on trying to write a new message, or reply to an incoming message, these defiant messages come back to fill the body of the message window. Where are they coming from? It's driving me crazy!

Hi Matt, the problem has inexplicably disappeared. No idea why, it's just gone. thanks again for your response. Steve.