Please can you read the post at the top of this section for forum with the title: Please read before posting in Disputes & Prosecutions
https://www.railforums.co.uk/threads/please-read-before-posting-in-disputes-prosecutions.77759/
In this post you will see it asks that you provide the following information so you can receive the advice accordingly: -
The stations where you started & finished your journey;
The stations where you changed trains (if applicable);
If you presented a ticket(s), the information stated under "Ticket type", "From", "To", "Route", and any other relevant details;
What happened in any encounter with railway staff;
The details of any paperwork with which you were issued.
Once you reply with this information you maybe able to receive more tailored advise. Also, perhaps you could advise what the letter you have received is saying exactly.
Once you spoke to the lady, did they give you the opportunity to pay an excess fare or did they go straight for asking for your name and address?