You don't need a wordy appeal in this case (
although if you did you should be able to attach a PDF with more detail). As alterego explaind in post #16, the appeal is very simple - you produced a valid ticket hence the Penalty Fare was issued incorrectly. There is no need to include the extraneous detail outlined in post #1.
The Regulations can be found here
https://www.legislation.gov.uk/uksi/2018/366/contents
The relevant parts of the Regulations in this case are
and
You produced a ticket as required by Regulation 4 and therefore no penalty fare may be charged under Regulation 5.
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People have highlighted various other reasons why, if a PF was apppropriate
(as explained it was't), the one you received could be successfully appealed but I see no merit in mentioning them as the simple fact is that no PF should have been issued in the first place. If you do want to add further reasons then, as previously mentioned, other posters should be able to suggest appropriate wording.