As an aside, it may be worth the OP keeping an eye on the related thread:
http://www.railforums.co.uk/showthread.php?t=126882 where another OP is planning to sue a TOC for recovery of a PF paid under protest under threat of prosecution despite (allegedly) holding a valid ticket.
This could be one potential course of action if this OP decides that the risk and hassle associated with not paying the PF is too great, which I would fully understand. The costs associated with losing in the county court would be less than those associated with any form of criminal conviction in the magistrates' court, and would not involve the possibility of a criminal record for dishonesty.
I believe that there could be a claim for rescission under the Misrepresentation Act 1967 or a claim for either rescission or damages (but not both at the same time) under the Consumer Protection from Unfair Trading Regulations 2008 (which prohibit misleading and aggressive commercial practices).
However, the OP would first need to exhaust all alternative avenues (such as the appeal body, Transport Focus and the TOC customer services) and send a valid letter before claim. It might also be prudent to wait six months before filing a claim, just in case the TOC decides to refund the PF and pursue a (vindictive?) prosecution instead.
In general terms, personally, if I was absolutely certain that my ticket was 100% valid, I would not pay anything extra, on the basis that:
1) the PF is invalid and unenforceable, and hence any so-called "administration fees" added following non-payment are equally invalid and unenforceable, since one cannot claim administration fees for chasing a non-existent debt;
2) as several members of this forum have found out, appealing is not guaranteed to be successful even when a ticket is wholly valid (the appeal body lacks independence, IMO), and is hassle in its own right;
3) whilst paying the PF is not an admission of guilt, it may be effectively treated as such by the TOC in that any subsequent unrelated ticketing dispute following payment of the PF would be treated more severely as a supposed "repeat offender";
4) taking matters further if the appeal is rejected will be enormous hassle and potentially involve considerable extra cost, particularly if I end up suing for recovery of the PF;
5) the TOC is presumably unlikely to prosecute a valid ticket-holder; if they do, I would be very confident in my ability to defend myself successfully and then to recover most of my costs from the TOC following acquittal, and would take a great sense of satisfaction from doing so;
6) in contrast to (3), the TOC would be less inclined to bully me in future following the presumed acquittal, and;
7) I consider that threatening a valid ticket holder with prosecution as a means of coercing him or her into paying an invalid PF, or other ransom demand, is likely an aggressive commercial practice under the CPUTR, and thus a criminal offence in its own right (one which, unlike blackmail, does not require
mens rea), and I am disinclined to give criminals my money for the sake of an easy life.
Even in this circumstance, I would not simply ignore all correspondence, but rather would appeal (if available; there is no appeal mechanism against Northern's £80 Penalty Fakes) as a means of showing reasonableness and attempting to resolve the dispute quickly and simply. At the same time, I would write to the TOC to explain in detail why my ticket was valid, and to politely tell them that, regardless of the IPFAS ruling, any alleged debt is denied, as is the commission of any criminal offence on my part. I would add that, having paid the correct fare before travel, and having presented a valid ticket on demand, I would not paying anything more without a court order to that effect, and that any prosecution or county court claim would be vigorously defended, with an application for costs if my defence succeeds.
However, this is not intended either as specific advice to this OP or as generic advice, since each circumstance will differ, as will the attitude of an individual OP towards risk and hassle.