Sorry this happened to you. I had a similar experience, only way more costly. I was back in the UK visiting and had arranged a week in London to see my sisters... found a first class ticket for me and kids from Newcastle to london was actually only a little more than the regular ticket for those dates, so I reserved it and was so excited as I had never been 1st class before, nice to get tea and coffee etc. The journey down was lovely.
On the way back the taxi I rang to take us to the ststion never showed. We waited 30 kins and nothing. Rang again, nothing. I had built almost 2 hours to get across london into my journey, but by the time the third taxi finally got us to the ststion, we missed the train by 10 minutes.
No matter, I thought. It won't be reserved seating, but i still have return tickets. It had been 5 years since I was last in england on trains. I had no idea there were now more than one train line operating the route.
Within 10 minutes of setting off, just after we were settled into seats, the collector came round. I handed the tickets over and he was just horrific. Acting like I was a criminal trying to defraud the company with fake tickets. He demanded I buy 4 tickets on the spot, and when I said ok, but could I move us to 2nd class because i would have never sat in 1sr if I had known my ticket wasn't valid... he refused, saying I either bought 4 full price 1st class tickets or would be kicked off with a fine at the next stop.
I had to ring my mum in tears as I didn't have 310 pounds in my account and she had to pay over the phone for me. It was utterly humiliating and even though my tickets probably cost more than the rest of the people paid in the carriage, I felt like poor scum and didn't even dare have the free tea when they came round, just cried most of the way back. Totally spoiled the trip and honestly it pretty much put me off returning home at all...
Why can't they be nicer about it? Would it have killed them to allow us to move to 2nd class, given we had proof of the misunderstanding? Is he on commission for selling the most top tier tickets?