I made a wee bit of a mistake
I just loaded over 200 short fiction stubs (bits and pieces of stories) in .doc format, and only when I tried to convert one of them into EPUB did I discover they needed to be in .docx format.
Simple answer I know is to delete the lot of them, go back to the original .doc files and resave them as .docx, then reload them into calibre and convert the lot of them to EPUB.
However, I wondered if there is a simpler process, maybe installing a plugin that DOES convert .doc files - assuming there is such a thing ?