My wife got PF'd on an Ashford International - Orpington service having travelled in on the Eurostar from Paris, not realising she couldn't just buy a ticket on the train (and oblivious to the signs at Ashford which told her that she had to have a valid ticket before boarding). An open and shut case as far as the rules are concerned.
We appealed on the following grounds:
1. PF was calculated on double the full fare from Ashford-Orpington, not from Ashford to the next stopping station, Staplehurst (which the train hadn't yet reached)
2. On that basis, incorrect fare used as split ticket Ashford-Staplehurst plus Staplehurst-Orpington was at that time about 40p cheaper than the through ticket, and valid because train stops at the station where changing from one ticket to the other
3. She was told she had to (and so did) pay the full amount straight away rather than just the fare element now and the penalty element within 28 days
4. SRA guidance at the time stated it was undesirable to levy penalty fares from international interchanges such as aiports as there was an increased risk of confusing people unfamiliar with the system
5. No opportunity to buy a ticket at any point between the international and domestic platforms at Ashford (i.e. no TVM) - need to exit and then reenter the barriers to use the booking office, not obvious to to the occasional traveller simply following the direction signs to the domestic services
6. The Penalty Fares warning poster on the domestic platforms was the same size as and displayed next to various advertising posters and didn't particularly stand out as an official communication
Our appeal was successful, but IPFAS didn't state why. I suspect point 1 was the main clincher. If the fare had been correct I don't know if any of the other issues woud have carried any weight.