I've thrown this together and would appreciate if someone who knows what they are doing can have a look at it and see if it will actually work. I've made it so that all one needs to do is drop a folder into their templates folder and it will take care of this worldpay problem.
This is not intended to be a permanent fix just a stop gap measure until Worldpay decides to pull their heads out.
See the following Screen captures of both the Success and Cancelled pages.





Since the mechanics of this module work great all we are bothered with is what the customer sees after they have either completed their transaction or cancelled it and this , i believe, is an acceptable outcome for those of us who are challenged

Because forum rules prohibit me from posting a link to this fix if you Philip or anyone else who wants to have a look at it will either PM me or email me and i'll send you a link to it.