Ok. After a frustrating weekend I have a workaround (but still not real reason why the simpler version worked on one install and not another!)
$args = array(
'posts_per_page' => $pageSize,
'offset'=> $offset,
'tax_query' => array(
array(
'taxonomy' => 'category',
'field' => 'term_id',
'terms' => 9
)
);
While this is a totally workable solution, I’m still baffled.