It looks to me as though the problem lies with the covers. It seems to manifest with the back cover, but when I shoogle back and forth on the back cover (via my iPad) it re-appears. Often then, the front cover has vanished instead.
I'm not convinced that this is anything to do with my html. I'm guessing that it might be related to my TOC, or possible .opf, or CSS.
Here's my TOC:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<meta name="viewport" content="width=1200,height=1700"></meta>
<meta content="text/html; charset=UTF-8"></meta>
<title>The very first day</title>
<link href="css/styles.css" type="text/css" rel="stylesheet"></link>
<img src="images/PeSplash01FCover2.png" alt="The very first day front cover" />
I'll have a look at the other files and paste them here if I think that might be worthwhile or useful to others.