None of the above worked for me - the problem lay in my Sourcetree Preferences. In the Network tab, I had a setting there for 'Default usernames for URLs which do not include one:'. The username was incorrect where I had entered it incorrectly previously - I had set it to my email rather than username. I highlighted the entries and clicked Remove for both. Then I returned to my repository page and clicked Push again. On pushing, it asked me for full username and password, which I was able to enter correctly - the push then finally worked.