By the way, I managed to resolve this myself but only because I had Updraft Dashboard installed on my lead blog. That was able to log me in to the problem blog (though direct log in was not working for me) and once in I deactivated and deleted the rogue from my dashboard – all good now but it does mean I heartily recommend Updraft as a saviour. May be worth considering having a totally clean extra blog installed just to host the Updraft software?
