Many thanks for the reply (again). I've managed to work around the problem by disabling the 'add multiple items to cart' button so i have an 'add to cart' button next to each item's quantity box and then using Ajeh's fix (here: http://www.zen-cart.com/forum/showthread.php?t=174212) to stop the redirection to the product info page when a customer adds something to their cart. This is working with the Single Listing Template as well as the main category pages (although i still need to give it a good test).
It's taken a little time to get things working and positioning as i needed but i think i've learnt a lot and now have things right. Fingers crossed a full test will not find any issues!