Found a wee bug...

In tpl_modules_categories_tabs.php

Line 26 - $cPath = explode('_', $_GET['cPath']); & subsequent line 33 using this exploded $cPath.

Setting $cPath as an array here caused a few errors... a couple of PHP Warnings about strpos() in main_template_vars.php on line 75, and trim() in functions_general.php on line 63, and it also meant the Manufacturer filter didn't work as $cPath wasn't getting passed as a hidden field.

Changing this to $cPath_ary = explode('_', $_GET['cPath']);, and replacing $cPath with $cPath_ary in line 34 seems to do the job. (I'd have used the more obvious "$cPath_array" but it's used elsewhere and didn't want to risk messing up some other code.)