I finally cracked this. I went on line with the Apple Support people. They very patiently went through all of the things I had done, then they were stuck. They suggested a couple of things which didn't work. Final solution was to get to a random log in web site, then before putting in any data like your email that his usually overwritten incorrectly, right (two finger) click on the blank "username/email address" field. This brings up a a selection box of which on option is Spelling and Grammar. Highlight this with your cursor and another selection opens up. Click on the "Show Spelling and Grammar" option and another box opens. This next set is a bit hazy (it was late, I was tired and also desperate for a solution) so bear with me....I had two goes at this and am not sure which one worked so try one then the other...
In the blank top line of the Spelling and Grammar box I put my Wrong ID and then clicked "change". Nothing seemed to happen so I put in the Correct ID and clicked "change". Again nothing seemed to happen.
However when I clicked out of all these boxes, and went to log in to a web page as a test, for some reason this seemed to have cleared the incorrect log in auto-fill. The Apple Support person, who had access to my screen by this time, was amazed and couldn't explain why what I had done had worked.
This was over a week ago and the erroneous log in/auto fill has gone for good.
Hope this helps someone. Apologies that I was't as methodical in getting the certain solution but as I say, try one, then if that doesn't work try the other. One of those should clear it.
I did ask the Apple person to make sure this fix is posted widely on all forums but I don't think they did.