Thanks for replying.
This is an example of Calibre's html:
<body class="calibre">
<p class="block_71">Maybe it’s the first morning that you wake up and notice the birdsong and the blue sky instead of a familiar sense of dread. Or it might be the moment after a laughter-filled chat with a friend when he or she says, ‘You know, you seem different. More like your old self.’ Or perhaps the world simply seems brighter, funnier, more inviting.</p>
And the corresponding css (I've changed the sans-font to Arial, which bizarrely fixed some of it, but not all.):
.calibre {
color: #000;
display: block;
font-family: "Book Antiqua", serif;
font-size: 1em;
padding-left: 0;
padding-right: 0;
margin: 0 5pt
}
.block_71 {
color: black;
display: block;
font-family: "Arial", sans-serif;
line-height: 1.2;
text-align: justify;
padding: 0;
margin: 0
}
And the attached screenshot shows how it appears in iBooks.