Why is my margin formatting lost in epub conversion?

Re: using Pages to make a file and export it as an epub.

I want to increase the top margin on a Pages file so that there will be a larger space between the header (author / title of the book) and the start of the text.

I have tried using the Text Inspector (Document Margin, Top) and all looks fine on the Pages document itself.

However, once the Pages document has been exported to epub format and I look at it in iBooks (on my iPad) as an epub, nothing has changed.

Something seems to be going wrong at the conversion stage, but I cannot work out what it is! Any ideas, please?