So, even though this is now a criminal prosecution, we can (potentially) negotiate a "deal" between two private parties (i.e. between us and the train operator). Is that correct? If so, we will try that.
In terms of appearing in court, is it worth fighting the case, or just pleading guilty to reduce costs etc. I guess this is not the first time this has happened to someone - are there any precedents here? It just seems very wrong to get a criminal conviction when she had a perfectly valid season ticket - but just forgot it for one day out of many years travelling on the train. Also, could you advise what kind of impact a criminal conviction like this has - e.g. on credit ratings, employment checks etc.