Issue resolved, see below - IE issue not the source
I tried to update my Epub copy of the Calibre Manual from the website today
(
http://manual.calibre-ebook.com/) but the link EPUB format in the line
"An ebook version of this user manual is available in EPUB format, AZW3 (Kindle Fire) format and PDF format."
downloaded a file named calbre.zip.
This file seems to contain mainly html files relating to the help topics and not the expected "calibre.epub" I had before.
The AZW3 download reads fine in Calibre.