Hi
Sorry for this careless info about W2X. Thanks for correcting me Pholy!!
The complete name for the extension is
writer2xhtml.oxt. You'll find it in a folder containing other very interesting items but that you do not need to install if you just intend to produce EPUBs. Just unzip this folder and look for it.
Current version is 1.1.7. The 1.1.8 will probably be published end of July.
The main idea is to use LibreOffice with styles (and style too !!) and then you just export to EPUB. I'll be happy to give you any info about it.