Set up the new Gmail account in Thunderbird. File (Alt-F) - New - Existing Mail Account

Thank you for that advice. That appeared to change the email address alright. But I have also changed internet provider and in my attempts I think I have changed another setting. So my email is not arriving to Thunderbird. Can I check something else? Thanks

That appeared to change the email address alright.

If you did what has been suggested you created a new account. There was nothing actually "changed".

I have also changed internet provider and in my attempts I think I have changed another setting.

If you successfully set up the new Gmail account in Thunderbird then there is nothing wrong with your Internet connection. Thunderbird doesn't care about your ISP as long as it works.

So my email is not arriving to Thunderbird.

That's not exactly a comprehensive problem description, you'd need to be more specific. Is there any error message? Also, please post your Troubleshooting Information. At the top right of the Thunderbird window, click the menu button ≡, then select Help > More Troubleshooting Information. Press the "Copy text to clipboard button" and paste the information into your reply.