Some suggestions, which if you feel appropriate you can drop into the appropriate sections of your letter.
(First para)
Further to your latest letter of x, I believe that, in line with the NRCoT, I was entitled to attempt to purchase a ticket on arrival at Bridgend, (which I attempted to do), and will again set out my journey and the reasons why I believe this. I think it is now well established that I commenced my journey at Parson St, a station without ticketing facilities, and that point is not at issue.
Between Parson St and Cardiff Central, the obligation is to buy a ticket "as soon as you are reasonably able" "from the conductor on the train if one is available". This is not the same as your assertion that I should "pay the fare due immediately on boarding the train". It is not unreasonable to assume that in a journey of over an hour a ticket check will be carried out, at which time I would have purchased a ticket. It is not reasonable to expect a passenger to seek out a conductor on a crowded commuter train, thus giving up my seat, particularly when the conductor is often in the back cab and unavailable.
At Cardiff, as the NRCoT explicity state that I should only purchase a ticket if sufficient time is available before my connecting service, again I did not breach them, contrary to your assertion, with only 4 minutes before the scheduled departure (less than the minimum connection time, and certainly less than time needed to find the unpaid fares staff, queue, pay, and return to the platform).
(Second bullet between Cardiff and Bridgend)
However, notwithstanding the discussion with the catering staff, by this time I did attempt to seek out the conductor, although as stated above there was no obligation for me to do so. I passed through the full length of the train but was unable to find anyone.
Personally I would drop the section regarding the confusion between Bristol and Parson St, although you might add to the appropriate bullet point "...to which I replied Bristol. As Parson Street is in Bristol, this was an accurate answer - had I been asked which station I had travelled from then I would have been more explicit in my response."
(Before last para)
I note that you quote the Railway Byelaws 2005 of the Regulation of Railways Act 1885. Setting aside that there is no such act, the relevant paragraph of the Byelaws is Paragraph 17, travel from a compulsory ticket area, where paragrah 3.1 specifically allows me to board at Parson St, as it has no ticket purchasing facilities.
Last para
In conclusion, I started from an unstaffed station, I was given no opportunity to purchase between Parson St and Cardiff, no opportunity between Cardiff and Bridgend, and so was entitled, under the NRCoT to purchase a ticket at Bridgend, which I actively sought to do, and was denied. I would note that the NRCoT explicity state that the option to purchase on arrival is allowed under these circumstances. At this stage I have still not been given a chance to pay the fare due, and would reiterate that I wish to do so to settle the matter. I am however, not prepared to pay the amount due, and should you proceed to a court case then I will be pleading not guilty and setting out the circumstances, as laid out above, which has led to the railway breaching its own Conditions of Travel.
This is definitely worth fighting. Hope these suggestions are helpful, and happy if others chip in with any other comments.