Thanks DE, I went looking on the forum but found this thread saying that 2.6.2 no longer processes Calibre created epubs.
https://www.mobileread.com/forums/sho...d.php?t=224230
The add-on would be fine if it was just my person use I was concerned about but I am about to release my book to the public domain and don't expect anyone to have to tweak their software just to be able to read it.
It looks like 2.8.1 has been released; but as I am in Australia we are usually the last to receive it. I'll guess that I'll wait to see if that fixes the issue.