Yes. Adjust the top margin while you are on the master page. Set this document to be the main document in the Books panel. In the panel menu button you can choose to synchronize that master page to the other 98.
Your answer confirms what I had thought might be possible, but which also seemed to simple and easy to be true!
I thought that perhaps only character, paragraph and object styles synch, that master pages were handled differently (I have only dabbled so far -this morning, in fact in page numbers and headings top/bottom each page indicating book name and the unit numbers, hence the realisation that I needed to increase my top/bottom margins).
Tomorrow I will set to work adjusting margins in the manner you have explained. Thanks again for your help!