It's not just the last page, it's also happening on the last DPS, and also the front cover.
It's an intermittent problem; so some times it works as it should, and other times I get blank pages.
What I don't understand is why the epub file passes validation. This means that I don't have any error messages to use as a starting point. So I don't know what files I should be looking at (should I be looking at the xhtml, the opf, the css?)