In the end the only solution I found was to go in to my Google account and turn off secure apps or more precisely swich to allow unsecure apps. As soon as I did this I can access my gmail account again OK but it does beg the question why does Google NOW consider the Thunderbird application to be insecure.

On your computer, go to gmail.com.; Enter your Google Account email or phone number and password. If information is already filled in and you need to sign in to a different account, click Use another account.; If you see a page describing Gmail instead of the sign-in page, click Sign in in … Sign in - Google Accounts Sign in - Google Accounts Gmail - Email from Google

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Can't access Gmail account Solved - Windows 10 Forums Feb 23, 2017

On your computer, go to gmail.com.; Enter your Google Account email or phone number and password. If information is already filled in and you need to sign in to a different account, click Use another account.; If you see a page describing Gmail instead of the sign-in page, click Sign in in …

Here's How to Log in to Multiple Gmail Accounts at Once May 22, 2020