Gmail Stopped Working - Google requires completing authentication in your web browser

Gmail has suddenly stopped supplying emails and won't connect. I've tried removing the Google email address from my Mac and then adding back in - It gives me a message "Google requires completing authentication in your web browser." but when I click the option to do this it just hangs there. I've logged in separately on Gmail and thats working fine. No option to do anything about it. I did note that it seemed to want me to open this Authentication in Safari but I've tried that to no avail. Any ideas please?