Hello Snarg,
I am trying to install the same contrib and am coming up against issues as well. I see this thread isn't supported but I thought I'd post and see if you came across this issue as well.
At step 12 of the instructions it says to replace this code in file catalog/includes/modules/product_listing.php:
$prod_list_contents .= ' <td align="center">' . tep_draw_button(IMAGE_BUTTON_BUY_NOW, 'cart', tep_href_link(basename($PHP_SELF), tep_get_all_get_params(array('action', 'products_id')) . 'action=buy_now&products_id=' . $listing['products_id'])) . '</td>';
Unfortunately this code line doesn't exist in this file! Did you come across this issue as well? You say you managed to install the contrib so I assume you must have found a way around this. Any help appreciated!