Custom Successful Payment page for Open Cart / WorldPay

WorldPay is already successfully set up in OpenCart shop (using v 1.4.9.4) and payments can be made. (The customer is taken away from the shop to make the payment through WorldPay.)
On completion of a transaction, what looks like WorldPay's own "Payment Complete" screen is displayed.
I would like a custom page to be displayed with a link back to the shop - i.e.
I would like the http post with the transaction info to then be output as a page of html with my own message and link back to the shop.

Completion of this job should include details of the custom page so that I can edit it in the future if my client wishes to change the "thank you" message.

Ideally I would like to make a decision on which bid to go with by the end on the day on Wednesday 7th March. Thank you.