Gemma, I think you need to realise that people here are trying their best to help you but you are not presenting yourself in the best light. If you displayed this tone on the day of travel and/or in your complaint afterwards it may have some bearing as to why the train company is considering taking you to court. You have 2 options open to you now, like it or not, they are: -
1. Respond to the letter apologising for what happened and admitting your mistake, assuring Northern that you shall purchase a ticket before you board the train in future and offer to pay their costs involved in dealing with this matter.
2. Do nothing and risk being taken to court by Northern, being convicted possibly ending up with the criminal record and receive a fine much higher then if Northern were to offer you an out of court settlement.
It is time now to either deal with this or not. Any grievance you may have with the railway system is, quite frankly, a moot point. Time to get serious now and deal with this. I would strongly advise you take option 1 above.
I am also curious to know, have both you and your partner received letters from Northern or just yourself?