Barnaby,
If "PDF_icon_15p" is not the name of an actual field that contain the string value that you want to concatenate with the iconprefix and iconsuffix then you need to more it all to the iconprefix or iconsuffix tags.
</link>
<link includeinresults="true" field="pub_other_pdf" alias="Other report link" tooltip="Link to other report" popuplinktype="image">
<linkprefix/>
<linksuffix/>
<iconfield></iconfield>
<iconprefix></iconprefix>
<iconsuffix>http://myserver/assets/icons/PDF_icon_15p.jpg</iconsuffix>
</link>