Calibre conversion worked well, (4 sec for a 250 page novel), and I think that I see what JSWolf means about the CSS changes: most of the styles end up named ".calibreNN" instead of carrying through the original style names. I'm not bothered about this as the book displays perfectly and the conversion process is so fast.
One thing I did notice when viewing the generated stylesheet.css is that it contains:
.calibre { font-size: 0.875em }
This is because I change the body style in my Kindle books to have this setting as I find the standard Kindle fonts are too big. Is there any way that I can force this back to 1.0em during conversion or will I have to edit the final ePub to reset it?