Order details not being sent using World Pay

We are currently using WorldPay as out payment gateway on our Zen Cart and all has been going well until now.

Yesterday someone purchased an order on our website and we received the email email back from WorldPay saying how much the order was for etc, but we never received an email from Zen Cart (as we normally do) detailing exactly what the order was.

Can anyone please help?

In the short term we can email the person back and asked what he ordered, but this would be a pain if we had to keep doing this on a regular basis.

To cast a bit more light on this, this is what error message we get when people use World Pay.

Home :: WorldPay
Warning: Cannot modify header information - headers already sent by (output started at /home/london/public_html/shop/includes/templates/template_default/common/html_header.php:22) in /home/london/public_html/shop/includes/functions/functions_general.php on line 44

Here is the offending line 44 in functions_general.php

header('Location: ' . $url);

Thread for World Pay module can be found at
http://www.zen-cart.com/forum/showthread.php?t=39106&highlight=world+pay