No Notjohn, we aren't talking about book ids. We are talking about ids generated when you split the file at the cursor. This generates a uid in the opf -- see below(in bold);
Code:
<manifest>
<manifest>
<item id="ncx" href="toc.ncx" media-type="application/x-dtbncx+xml"/>
<item id="styles_css" href="Styles/stylesheet.css" media-type="text/css"/>
<item id="cover" href="Text/cover.xhtml" media-type="application/xhtml+xml"/>
<item id="contents" href="Text/contents.xhtml" media-type="application/xhtml+xml"/>
<item id="Section0002.xhtml" href="Text/Section0002.xhtml" media-type="application/xhtml+xml"/>
<item id="cover.jpg" href="Images/cover.jpg" media-type="image/jpeg"/>
<item id="body1" href="Text/Chapter_1.xhtml" media-type="application/xhtml+xml"/>
<item id="body2" href="Text/Chapter_2.xhtml" media-type="application/xhtml+xml"/>
<item id="body3" href="Text/Chapter_3.xhtml" media-type="application/xhtml+xml"/>
<item id="body4" href="Text/Chapter_4.xhtml" media-type="application/xhtml+xml"/>
<item id="body5" href="Text/Chapter_5.xhtml" media-type="application/xhtml+xml"/>
<item id="body6" href="Text/Chapter_6.xhtml" media-type="application/xhtml+xml"/>
<item id="body7" href="Text/Chapter_7.xhtml" media-type="application/xhtml+xml"/>
<item id="body8" href="Text/Chapter_8.xhtml" media-type="application/xhtml+xml"/>
<item id="body9" href="Text/Chapter_9.xhtml" media-type="application/xhtml+xml"/>
<item id="body10" href="Text/Chapter_10.xhtml" media-type="application/xhtml+xml"/>
<item id="imag25849" href="Images/image001.jpg" media-type="image/jpeg"/>
<item id="imag70213" href="Images/image002.jpg" media-type="image/jpeg"/>
<item id="Title.xhtml" href="Text/Title.xhtml" media-type="application/xhtml+xml"/>
<item id="12c14ef3-e5d0-4c7f-af43-9a5fa134c2bf" href="Text/Section0001.xhtml" media-type="application/xhtml+xml"/>
</manifest>
<spine toc="ncx">
<itemref idref="cover"/>
<itemref idref="contents"/>
<itemref idref="Title.xhtml"/>
<itemref idref="12c14ef3-e5d0-4c7f-af43-9a5fa134c2bf"/>
<itemref idref="Section0002.xhtml"/>
<itemref idref="body1"/>
<itemref idref="body2"/>
<itemref idref="body3"/>
<itemref idref="body4"/>
<itemref idref="body5"/>
<itemref idref="body6"/>
<itemref idref="body7"/>
<itemref idref="body8"/>
<itemref idref="body9"/>
<itemref idref="body10"/>
</spine>
...The above generated id ref will fail EpubCheck because the first letter is a numeric char. The first char must be an alphabet char to pass EpubCheck.