Ok I'm stuck, I've looked at some other questions on here but no help, I've searched online, I've logged into my test account but I can't find any documentation anywhere.
Does anyone have links to the WorldPay documentation, specifically when a 'Payment Response' is made, what format is it in?
The documentation can be found at http://rbsworldpay.com/support/bg/index.php?page=guides&c=UK
Sounds like you need the 'Payment Notifications' guide,
The Payment Response is an HTTP POST and is sent when a payment is Authorised or the user clicks 'Cancel' on the payment page.
You can find the set of data sent in this under 'Reference', 'Payment Message', 'Parameter Descriptions' in the Payment Notifications guide on the above page.