I would say that the major problem, especially with a company as big as First Great Western is that there is little communication between staff. The member of staff who fined you was really only doing his or her job.
I would advise you to write to First Great Western, explaining the situation and that you feel the current situation is unjustified.
Revenue Protection and Prosecutions
First Great Western
Freepost SWB40576
Plymouth
PL4 6ZZ
If they fail to satisfy your problem, then write to the Rail Focus Watchdog. You can contact them via
Passenger Focus - Putting Rail Passengers First or write to them at
Passenger Focus
FREEPOST (RRRE-ETTC-LEET)
PO BOX 4257
Manchester
M60 3AR