Beware the adobe xpgt file (
https://wiki.mobileread.com/wiki/XPGT); this is not part of the epub standard but is used by a lot of publishers. It also defines page margins. You can use Sigil to edit the margin-top in the xpgt file - if there is one Sigil puts it in the Misc folder in the left hand tree view. If you are feeling brave you can delete the xpgt file altogether, and use the CSS instead, AS WAS INTENDED IN THE EPUB STANDARD.