Thanks Cleverbeans, I was at the point where I was going try something like that, or even deleting the file from TW completely and Sharing it again....
Anyways, what happened in the end was I just kept trying to Receive Changes, each time it would tell me I had to restore from autosave. So I would.
Eventually ArchiCAD/Graphisoft must have realised something was wrong, and a message appeared asking me to send the file to them. I tried doing that but it kept telling me I was using the wrong username and password, so I gave up on that........
Anyways, after that I went back to open the file and receive the changes and now it is working fine!!!
Who knows!!!
Mike