Thanks for your reply.
The fact that there is a section called 'Debugging eCart Transaction Failed issues' clearly means it hasn't been fully supported. From a user's point of view it is extremely time-consuming to follow your instructions then to have to spend most of the afternoon scratching my head trying to figure out why it didn't work... then to find out later after submitting a ticket that there have been issues... therefore the SagePay solution hasn't really been fully supported - just check out the number of users with errors - my point is that I shouldn't have to spend time getting these extensions to work when your PDF walkthroughs fail.
When I first purchased a WebAssist extension package - Super Suite - a couple of years ago, I was sold by Mark Fletcher who promised me that Protx was definitely supported... only after making the purchase I realised this was not the case and I then had to submit a support ticket to have a custom solution worked out for me... ever since then there have been issues.
I admit that yes things are getting better and more compatible, and of course I appreciate all the hard work your team are putting in, especially with the change from Protx to SagePay, so please keep up the development and support, but just try and check your PDFs before publishing!