Unfortunately, for a byelaw 18 conviction all SWT need to prove is that you didn't have a ticket for your journey, which you didn't according to their RPI's statement. Were you questioned under caution? What was said, even if not under caution?
If a staff member let you through at Norbiton, it could be seen as getting authority to board the train without a ticket, but I'd imagine this would have been bottomed out during questioning? To be honest, it's unlikely a case would have got this far unless all the Is were dotted and the ts crossed....