How about then:-
Further to your previous letters, I still don't understand why you think my ticket was not valid. The return part of my ticket (an off peak return) was valid for one month from the 28th September, and it was being used, for the first time, on the 29th. I've not been able to find any terms on conditions, either in the National Rail Conditions of Travel, nor the omio terms and conditions, that required me to validate the ticket only on the day of travel. Indeed, I understand that the ticket can be used for a break of journey over several days, which if what you say is true regarding validation of the ticket, would mean anyone breaking their journey in that way would be travelling invalidly. Surely that would be an untenable position for the rail industry to take?
In my previous letter I offered to make a settlement, as, based on your letter, I was led to believe that it was my mistake to have validated the ticket at the same time as my outward journey. Having now investigated the situation further and taken advice, I don't believe you have any grounds for taking action against me, and so am withdrawing my offer of a settlement. I therefore ask that you confirm that you are dropping the case against me, but if you continue then I shall be obliged to contest the case in court, on the grounds stated above.
Someone ought to test this out, and then go to the media (eg You and Yours, Moneybox, on Radio 4), to force the industry to clarify the position and compensate anyone unfairly fined to date, in the same way that the industry was forced to do with incorrect calculation of penalty fare amounts recently.