Rewriting a custom post type slug issues

Hi,

I’m trying to rewrite a custom post type’s slug so that its taxonomy term appears first. So for example:

post type: case_studies
taxonomy: work
terms: commercial, domestic

so i want the url to end up being: website.com/domestic/case_studies

Why does the code below give me a 404? I can make it case_studies/domestic easy enough but not the other way around.

Apologies if this is simple stuff, quite new to this aspect of wordpress.

Pete

add_action( 'init', 'cptui_register_my_cpts' );
function cptui_register_my_cpts() {
	$labels = array(
		"name" => "Case Studies",
		"singular_name" => "Case Study",
		);

	$args = array(
		"labels" => $labels,
		"description" => "",
		"public" => true,
		"show_ui" => true,
		"has_archive" => true,
		"show_in_menu" => true,
		"exclude_from_search" => false,
		"capability_type" => "post",
		"hierarchical" => false,
		"rewrite" => array( "slug" => "%work_type%/case_studies", "with_front" => false ),
		"query_var" => true,
		"supports" => array( "title", "thumbnail", "page-attributes" ),
		"taxonomies" => array( "work" )
	);
	register_post_type( "case_studies", $args );

// End of cptui_register_my_cpts()
}

add_action( 'init', 'cptui_register_my_taxes' );
function cptui_register_my_taxes() {

	$labels = array(
		"name" => "Work Type",
		"label" => "Work Type",
		);

	$args = array(
		"labels" => $labels,
		"hierarchical" => true,
		"label" => "Work Type",
		"show_ui" => true,
		"query_var" => true,
		"rewrite" => array( 'slug' => 'work_type'),
		"show_admin_column" => true,
	);
	register_taxonomy( "work", array( "testimonial", "case_studies" ), $args );

	unset($labels);
	unset($args);

// End cptui_register_my_taxes
}

add_filter('post_type_link', 'work_types_permalink_structure', 10, 4);
function work_types_permalink_structure($post_link, $post, $leavename, $sample) {
    if (false !== strpos($post_link, '%work_type%')) {
        $work_types_type_term = get_the_terms($post->ID, 'work');
        if (!empty($work_types_type_term))
            $post_link = str_replace('%work_type%', array_pop($work_types_type_term)->
            slug, $post_link);
        else
            $post_link = str_replace('%work_type%', 'uncategorized', $post_link);
    }
    return $post_link;
}