Add the response URL callback required in to the code so you aren't relying on the user entering the correct URL

As said here on the RBS WorldPay website (http://www.rbsworldpay.com/support/kb/bg/paymentresponse/pr5501.html)

In the admin screen it tells you to add your URL as the payment response URL which users could well get wrong, so we could dynamically add the URL based on the base path.

A check could be made to see if the Secure Pages module is installed and change the http:// to https:// and add the /cart/checkout/complete URL to the Secure Pages enabled URLs.