ok I now have a pretty looking website (still an issue with Google Ads displaying, see below in case there is a connection.)
The prev/next navigation doesn’t show yet I found this in the code.
</header><!-- #masthead .site-header -->
<div id="main-wrapper">
<div id="header-featured-image"><a title="Portraits" href="https://www.portraitcorner.co.uk/" target="_self" rel="noopener noreferrer"><img alt="Portraits" src="https://www.portraitcorner.co.uk/wp-content/uploads/portraitbanner-1.jpg" /></a>
</div><!-- #header-featured-image -->
<div id="main">
<div id="content-sidebar" class="container">
<div id="primary" class="content-area">
<div id="content" class="site-content" role="main">
<nav role="navigation" id="nav-above" class="site-navigation post-navigation">
<h3 class="assistive-text">Post navigation</h3>
<div class="nav-previous"><a href="https://www.portraitcorner.co.uk/turkish-van-cat.htm" rel="prev"><span class="meta-nav">←</span> Turkish Van Cat</a></div> <div class="nav-next"><a href="https://www.portraitcorner.co.uk/portraits-from-photographs.htm" rel="next">Family Portraits from Photographs <span class="meta-nav">→</span></a></div>
</nav><!-- #nav-above -->
May or may not be part of same issue if faulty includes or something similar:
1. I have 18 sites with Advanced Ads for Google Auto Ads. All are working perfectly except for this site. It displays ads on mobile but not on desktop.
2. It’s clear that when I find a theme that doesn’t display prev/next links, they are still there, just not displayed.
3. The theme I am currently using displayed an old menu in the header until I re-saved the menu then it displayed proper navigation. Again I don’t know where this old navigation comes from.
Any and all ideas where to look would be very welcome.