Advice taken, except it seems I have run out of IQ.
However, with the kind of pioneering spirit that made this island nation mediocre, I had another bash using another PowerGallery installation on another site.
My idea was to check the process at each stage. So having checked that my one test product displayed fine in its detail page:
Stage 1. I uploaded the wa_evaluate.php file and changed the code in Products_Detail.php as before, from
<div id="ProductLongDesc"><?php echo $row_WADAProducts["ProductLongDesc"]; ?></div>
<?php require_once('wa_evaluate.php'); ?>
<?php echo wa_evaluate($row_WADAProducts["ProductLongDesc"]); ?>
I then tested the product detail page. It is now broken, as in the other store.
Stage 2. I abandoned all other stages.