Looked into this and its describing the editing of the worldpay payment page rather than the results page. The results page is the last worldpay page displayed and this is where I believe the redirect should take place.
It seems that you have to download a worldpay success file called resultY.html then add a meta-refresh or a static redirection url and upload the amended file back again. Does this sound familiar to you? The worldpay documentation is not user friendly.
This can redirect to a "thankyou for ordering" type page to display to the customer.
When the customer is redirected to this page does any code need to be added to the page to clear the cart, if so, what is it?
A manual redirect to the shop after the worldpay results page still shows the cart contents.