bnm, I know that you shouldn't have to carry anything with you to prove it is valid but have you received anything officially in writing from the train company confirming it is a valid route?
When I had issues with Scotrail last
year that were eventually resolved I took a copy of the email I received from the Scotrail as well as the letter I received from their Customer And Stakeholder Liaison Manager with me the next couple of times I made the same journey.
One one occasion I was told I'd have to pay and when I asked if they were sure and they said yes I took out the letter and email and the station staff decided that they were in the wrong and let me through.
I've had no problems since I produced the letter.
I understand that Paddington is a much bigger station with many more staff than at Aberdeen but could taking something with you help you from any further delays and missing trains or will they think that you've made up the letter/email yourself and continue to get it wrong despite the supposed rebriefing?