Can I respectfully suggest that this is not the best attitude to take? Northern could quite reasonably point out that it wasn't them who failed to buy a ticket. And we don't yet know if they failed to communicate to your proper address or if somehow you failed to receive it. If you are successful in getting the conviction struck out, you will want Northern to co-operate with you in agreeing a more favourable settlement. So working on the basis that everything is their fault is not going to encourage them to want to help you. Concentrate on what has happened, rather than why, and this may work in your favour.