It seems it is that the Bookbox and now also my Bookeen Orizon doesn't recognise quotes (double apostrophe) and apostrophe characters that appear in the epub created by Calibre. Presumably because Calibre doesn't understand the characters used in the source mobi.
Calibre's viewer presumably doesn't recognise the characters either but it just ignores the unknown characters rather than inserting a ? which just messes up the grammar.
Again any ideas of what to do to correct this would be most welcome.