Thanks for that charleski and Jellby.
I've had a close look at the file but I just can't see any link that doesn't match up. In the NCX I have...
<navPoint class="chapter" id="932a1cb0-ec5c-4a75-ac02-9ef00e71d591" playOrder="58">
<navLabel>
<text>Also available</text>
</navLabel>
<content src="OEBPS/split_017.xhtml#also-available"/>
</navPoint>
...and in that file I have a...
<p class="also-available-title" id="also-available">Also available</p>
Don't normally have problems with broken links but I must be missing something obvious here?
Still seems odd that the file validates in epubcheck but is rejected by iTunes Producer.
Best
Michael