Reminds me of my nightmare with First Great Western. Tried to pay for an upgrade on a season ticket to First as there were seats available (normally standing room only throughout the train).
Other commuters with me had done this many times before and I even had a letter from FGW saying this was okay as I thought I should check. Was charged with fare evasion and "held" at Reading platform for 2hrs in sub zero temperatures, wasn't allowed to go to loo/get drink, called police but they couldn't/wouldn't help, FGW letter was ripped up by inspector (nice Mr XXXXX ).
Inspectors concocted story that I had been abusive and threatened them - suggested they check the cctv..... Ended up having to engage solicitor to fight court case. Cost £900 which my company paid. Was told that I should pay for upgrade before getting on the train - I pointed out that I wouldn't know if there was a seat available before getting on the train as I was normally paying £450/month for the pleasure of standing up as there were rarely seats between Reading/Padington!
I will never travel wth FGW again and now a group of us have a taxi to drive us to London - cheaper than the train and I won't have to see Mr XXX again as I'd probably not be very polite to him......