Thank you for your reply and that makes perfect sense. I was able to follow your instructions and disconnect my account from Google login. I also revoked access to all devices including the web client) and I can now see no reference to Google in any of my settings. I was logged out automatically of course when I revoked access to the web client which I was using at the time.
Unfortunately, it still didn't work. However, this prompted me to try something else. I tried to login using my Gmail address as the user name. That didn't work as the account password had to be reset but I went through that process and was able to log into another Evernote account that had no content. I have no idea how I was able to set up one account with my Gmail address as the username but able to log into another account using the same address but the Google login API. I don't even know that I did do this as there were no notes or login history but I deactivated that account and after a lot of faffing about, I have now been able to change my email address and all is now well.
Thank you for taking the time to help, I appreciate it.