It looks to the customer as if they have returned to our site, but if you look at the URL in the browser it is WorldPay. Because of this, Google analytics does not register the transaction:
Is that not what I said?

By which I mean that the page after the customer enters their details ssuccessfully is generataed by the worldpay server rather than your server.
Interesting idea to use a meta tag to redirect back to the site. I had tried a javascript redirect but that did not work. The code listed will not work because it is for a different software and in a different language. For Zen you will have to adjust includes/modules/pages/checkout_success/header_php.php

It is not a one line answer to the changes that need to be made.