Ignoring the weird wording of the latest letter (what does "extend the law" mean?), unfortunately your account explains exactly why you aroused suspicion. You repeatedly said that you had travelled from Bristol, and the staff member quite reasonably said that you had an opportunity to pay there. Under caution you then changed your starting point to Parson Street, which (conveniently) means you had no means of paying.. An inconsistent story is bound to arouse suspicion, especially when you change it when challenged. I understand from your account that you travel reasonably regularly from Parson St to Cardiff, and would assume that you know that the station is not called Bristol Parson St. Please be aware that I am not doubting your story, merely pointing out how it might seem to a staff member who is used to inconsistent stories.