Epubcheck  complains if the navpoint ids  in the toc.ncx begin with a digit.
e.g:
	Code:
	<navPoint class="chapter" id="23b2a4fa-6968-47cd-a55b-68ec2650ccde" playOrder="3">
      <navLabel>
        <text>A Reminiscence of Dr Samuel Johnson</text>
      </navLabel>
      <content src="Text/story05.html"/>
</navPoint>
 I've looked and can't find any other reference to this id in the ncx, opf or any xhtml file, so I assume they are arbitrary and only need to be unique.
So I just prepended an X to all the ids and it passed epubcheck and seemed OK.
	Code:
	 id="X23b2a4fa-6968-47cd-a55b-68ec2650ccde"
 What are these ids used for anyway?