I was caught by revenue protection. I offered to pay the fare, but they asked for my name and address. A few days later I received the penalty 'fine' paperwork in the post. As far as I can remember it was pay up or go to court. I paid up. It was only £20.00 (I think).
Just remembered now...
Similar thing happened to me on a train from Nottingham. My ticket only covered part of the journey. I paid the penalty, couldn't be bothered with going to court etc.