I was firstly let down by the system on 2 separate occasions by the ticket officers at both Wycombe and Marylebone who either advised me incorrectly or failed to follow procedures.
Agreed. The first one hasn't followed procedure, the second one effectively does not believe the first one would be so daft as to authorise travel, so reports you.
This left me in the sense that all I would had to do should anything be followed up was to prove I did have a ticket to the relevant authorities as I had done so with the 2 ticket inspectors via receipt and bank statements both of whom had acted reasonably satisfied with my explanation.
The second one clearly was not satisfied, because they reported you. They might have seemed genial at the time - they work in roles where people can blow up at them - but if they were satisfied you'd been given authority to travel, they would have no grounds to report you.
A letter was then received regarding a fine. I went on to explain the situation that occurred. This then turned into a dispute around being able to produce the actual ticket, the contents of follow up emails suggested that by producing the actual ticket then the case would be closed.
The train company wants the ticket, and not unreasonably so, to be fair. You were written up for not having a valid ticket having left it at home, and so it would have made sense for you to keep it. Hindsight is 20/20, but it seems odd to dispose of it with that hanging over you. A court might ask why you'd throw it away. You have a receipt for a purchase, but not the ticket. The company wants to satisfy itself that you have the ticket, could not have sold it on, or bought it for yourself for a future date, or refunded it or any other number of things one can do with a paper ticket. That's why the actual ticket is important. Notably, producing the ticket after the fact does not mean you haven't committed an offence (your defence here is actually that you were given authority to travel by staff). Chiltern seem to have been looking for grounds to cancel proceedings based on the idea that, to be honest, if you'd done the "proper" thing and
bought a ticket for the day and refunded it on production of your season ticket, as per the conditions, no harm done to them.
For me this was all still a dispute around whether I had actually bought a ticket and I was not going to pay a fine when I had proof in the form of receipt and a bank statement. Again the case went quiet for 3 months until the SJN arrived charging me with 18(1) and 18(2).
Ok, so, they will have offered to settle out of court. This isn't a fine (although it will feel like one!) and, while we haven't seen anything that was said, this - at heart - isn't a dispute about whether you have bought a ticket or not, it is whether you boarded a train without it and whether you have a defence to that; the defences are narrow and laid out in the Bylaw itself.
Your issue is quite simple. You need to prove to a Court's satisfaction that you were given permission to board that train and will have no evidence at all to substantiate that other than your own word. This is not about what is true and what is not; it's about what evidence you have to support your defence, which is, in fact, almost none I'm afraid. (For what it's worth, we see a lot of people here telling fibs, and on balance, I am inclined to believe your account!)
Chiltern merely need to show that you had boarded the train, could not show a valid ticket, and will even introduce your own statements where you will repeat the material fact that no, you did not have a valid ticket, against you - as evidence. This is very easy for them to do. The burden is on you to evidence your defence.
Now, some people enjoy the combative nature of going to court, and frankly I have no time for the train companies who I think should wind their neck in most of the time, and I like to see them being given a bloody nose now and again. But if your reputation is what is important here, along with the sheer financial cost of being found guilty, I think it would be remiss of us not to advise that the cards are not stacked in your favour here. You might win in court if you get a bench who like you and are compelled by you, but there is certainly no guarantee of that and I think you have less than a 50% chance of winning. If it were me, I would settle for the amount they offered, and instead appeal to my MP and the press. It will feel rotten for a while, but if the principle matters, you still have other avenues available to make it good later.