@Mister L...I've just downloaded the plugin again and tested it on 3 different epubs and it worked fine. It uses the file headings from the toc.ncx file to find the xhtml file headings. When I tested the plugin, it always found the xhtml headings and always inserted the title attribute with the correct NCX heading name without a problem.
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Things to try:
1. Download the plugin again. I did hurriedly change it a couple of times after initially uploading over small things that I wasn't happy with in the plugin. So I changed the plugin and re-uploaded several times. Trouble is I don't know
when you downloaded the plugin. So best to download the plugin again to be sure that you have the latest working version. I'm sorry about that but I was tired of making new plugins versions for every small plugin change.
2. Before you run the plugin you should
always generate a new epub NCX file by using
Tools > Table of Contents > Generate Table of Contents. Doing that will make sure that all the appropriate and correct html headings(containing any h1 to h6 tags) will be in the toc.ncx file. Then after that try running the plugin. Basically what I'm saying here is that if you have no heading names in your toc.ncx file then the plugin won't be able to find any headings in the xhtml files. Just to also mention that if your epub has no epub TOC page then it's also quite possible that your epub's NCX file will contain no heading names as well.
3. Last point, be sure that you don't have any double quotes or apostrophe's in your NCX headings. In test I found the ''' html entity in an NCX heading name with an apostrophe. That html entity will prevent that heading being found using my plugin. I got this problem despite having no entries in
Edit > Preferences > Preserve Entities(which surprised me). Also neither the
Reformat > Mend.. nor
Mend and prettify... will fix that html entity problem in the NCX file but you can fix it easily enough by manually deleting the html entity and re-typing the apostrophe as proper text into the relevant NCX heading name.
...And I promise no further plugin changes until after I get your next reply...