Thanks, Fruhulda for your help. In the end I found a solution to the problem, myself: trawling through the mountains of info from KDP, I found they recommend mounting image files as JPEGs rather than PDFs. By converting the PDFs I'd been using to JPEGs, I lost the unwanted blank pages. But please be aware if, like me, you use Calibre as a conversion tool to view your work: if I open the Calibre eBook, blank pages are can still be encountered, but once converted, none remain.
PS As for the section breaks, I use them because I need section endnotes after many of my chapters. Otherwise a simple page break would be, indeed, sufficient.