What is the size of your Sent folder? What is your anti-virus software?

Have downloaded and reinstalled.

You've been wasting your time. All your data such as messages, passwords and user preferences, and changes made while you use Thunderbird, are stored in a special folder called a profile. Your profile folder is stored in a separate place from the Thunderbird program, so that if something ever goes wrong with Thunderbird your information will still be there. It also means that you can uninstall Thunderbird and reinstall without losing your settings and data. And you don't have to re-install TB to troubleshoot a problem with the profile.

I have reset up my account which is IMAP.

Not sure what 'reset up' means.

Also tried 587.

There is no point trying arbitrary combinations of server settings. Just use the settings your email provider requires.

I use AVG which has not caused problems as has used this for years with TB.

Try to start Windows 10 in safe mode with networking enabled.

Does the problem go away?

Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.

Hi Christ1 Size is 1.3 GB and use AVG Professional and Malwarebytes. Reset meant I removed account on TB and then reinstalled it with my email and password and checked that TB had it as an IMAP account and not POP3. Will now try it with Safe Mode. Do you recommend any other antivirus as I have used AVG for as long as the early 2000's and not had a problem. Thanks Kind regards Alan Jones