I found the answer to this problem myself. The same night I posted here I was lying awake worrying about this - not sleeping - when it hit me - the site I had uploaded my Word doc files to is only for checking epub files. Every time a Word doc is uploaded to this site it gives the error messages I received - because it is not designed to check Word docs, only epubs.
It wasn't actually an error message from Smashwords, but from a site I thought was linked (I had been given erroneous information form someone elese) to SW. It isn't. So SW was not to blame. The person who sent me the web link was, as well as me for failing to understand what was posted on the site in the first place. Had I understood better what was being said I never would have uploaded a Word doc there. But that was what I had been told to do and I had done it.
Again - DOH!