For prosperity and future generations, this turned out to be quite easy.
The Joomla code needed to get the articles which were of a specific type, in my case accordion content types:
$db = JFactory::getDbo();
$query = $db->getQuery(true);
$query->select($db->quoteName('id'));
$query->from($db->quoteName('#__cck_store_form_accordion_content'));
$db->setQuery($query);
$accordions = $db->loadAssocList('id','id'); //puts all the cckIDs related to each article in the category into an indexed array
Then when looking at the category object I could see the ::cck ID:: code in the fulltext element (not sure why, I thought it was always stored in introtext but this may have been a little misconfigured by supporting site builder.
So to get the cck item ID
$cckID = ''; // BW filtering Accordion Content Type
preg_match('!\d+!', $item->fulltext, $cckID); // puts each cckID related to each article in the category into an indexed array
The the filter was simple, thie following if statement wrapped around the "leading items" output did the trick:
if (in_array( $cckID[0], $accordions )) { // filter checks that the cckId of the item is in the $accordions list from above. ?>
It's not pretty I know but for us to get this project moving without having to redo layouts and overrides etc, it was the simplest option.