Hi Later Alligator
I'm sorry that you find yourself in this position. I've not heard of this happening before. I don't understand why they are saying that they have no proof of you having an assessment if you have a decision letter.
I would ring the DWP and see if you can talk to a case manager.
Your other option is to cancel the appointment and say that you need a new date. That would give you more time to get it sorted out. This is difficult because you only have a couple of hours to get through.
Sorry, can't think of anything more you can do at such short notice.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems