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samcole - If you are going to be doing frequent DOCX to EPUB conversions then I suggest you would make your life easier if you find out why your DOCX First Paragraph style is not getting converted.
If you change the coding of an EPUB that was created by conversion (from anything, by anything) you might as well throw the conversion input away.
I convert 10-20 relatively simple DOCX's every day (with calibre) and I have no problem with it at all. Here's the essentials of my FirstPara style
BR