Change the "Out of Stock" Text on Main Shop Page

Hi,

I’m a bit stumped on something… how do I change the “Out of Stock” text on the main shop page? We are selling 1 off pieces, and “Out of Stock” doesn’t sound right.

http://london-calling.com.sg/shop/

I have added this to my functions:

add_filter('woocommerce_stock_html', 'change_stock_message', 10, 2);
function change_stock_message($message, $stock_status) {
    if ($stock_status == "Out of stock") {
        $message = '<p class="stock out-of-stock">Sold</p>';
    } else {
        $message = '<p class="stock in-stock">Available</p>';
    }
    return $message;
}

Which works for the single product images, but not for the main page.

Any help would be much appreciated