Okay, I've just cracked it, thanks everyone
This is what I did - and now I have the formerly kepub only ebook as an epub in Calibre:
1. As Peter advised above, I went to the following folder/file on my PC:
C:\Users\user_name\AppData\Local\Kobo\Kobo Desktop Edition\kepub
2. In that folder I noticed that all the kepubs had the same file date modified, today's date, which is, I assume, because Kobo Desktop re-loads them each time it's opened.
3. So I started with the first and last files and renamed them .epub. That is, I just added the extension .epub onto the end of the existing file.
4. Then I imported the files, one at a time, into my Sony Reader for PC which is my pre Calibre library for all epub books.
5. My book wasn't the first book, it was the last book - which I guessed means that the last book showing on that file listing is the most recenlty purchased.
6. I opened the book in Sony REader for PC and it was operating sort of slowly - but working and looking like it totally should look. Yippee
7. I imported the now epub book into Calibre, where Alf did it's thing, and opened it. It works perfectly
Yipppeeee ......
