Welcome to the forum!
I have blatantly stolen the below excellent advice from
@Hadders because it is excellent.
Travelling with an invalid ticket is a criminal offence and GTR are within their rights to prosecute you in the Magistrates Court should they wish to do so. The good news for you is they are usually prepared to offer and out of court settlement to people who co-operate with them and who haven't come to their attention before.
What will happen is that GTR will write to you asking for your version of events. It is very important that you reply to this letter. You might want to mention the following in your reply:
- That you are sorry for what has happened
- What you have learned from the incident
- That you are keen to settle the matter without the need for court action
- Offer to pay the outstanding fare and the train company's administrative costs in dealing with the matter
If you have done this before, particularly using the same "Trainline" or other account. GTR will be able to search your online ticket purchase history and see it. You will need to be careful how you reply to their letter. You should answer the questions they ask for truthfully but you are not required to incriminate yourself either.
If you are offered a settlement the cost tends to be around £150 plus the outstanding fare. Expect the outstanding fare to be charged at the full Anytime fare with no account made for what has alredy been paid. An out of court settlement might appear to be a fine, but it isn't and you won't have a criminal record as a result of accepting one.
Feel free to post a copy of the letter once it arrives (with personal details redacted) along with your draft reply in this thread and forum members will be happy to proof read it for you.