As I thought appeal unsuccessful by final independent appeals panel - justification as follows.
“Enquiries by the panel have found the calculation to be correct. The Regulations quoted by you in your third appeal refer to the "full single fare applicable". This is an undiscounted fare. Section 1.3 of the acompanying Penalty Fares Guidelines 2018 go further and state The amount of any Penalty Fare is £20.00 or twice the cost of an undiscounted full Single, applicable to the person travelling, from the station you started your train journey to the next station served by the train, whichever is greater. Although your comments about the penalty fare calculation have been noted, the value of a National Rail penalty fare is set by the government, not by the rail operator.”