If all the subtitles are <p class="calibre8" id="filepos######"><span class="calibre6 bold">
With other paragraphs not using those.
If they are you should be good to use this for search (regex mode)
Code:
<p class="calibre8" id="filepos\d+"><span class="calibre6 bold">(.+?)</span></p>
Replace with this
If not we'll need more details of what the defining part is that means that it's the subtitle and if any other paragraphs use that as well.