The OP and companions have apparently already paid the full Single fare for the journey in "part payment of the PF", and RPIs on this forum and elsewhere have said that there is no possibility of prosecution in these circumstances, and minimal possibility of being pursued civilly, particularly when there may be an arguable case that the PF was not validly issued.
Are you accusing RPI and Stigy of posting false advice in the other thread at:
http://www.railforums.co.uk/showthread.php?t=69065, and, if so, on what grounds do you claim that their advice is incorrect?
The "strategy" promoted by the
Evening Standard to avoid paying PFs (which I do not advocate or endorse), can, of course be easily thwarted, since RPIs are never under any obligation to offer a PF (and may instead report for prosecution), and, if a PF is indeed offered, are under no obligation to accept part payment (and, if a nil-paid or underpaid PF, as defined here as less than the Anytime Single, can then cancel and prosecute for non-payment).
Having said that, in this specific circumstance, I too would not advice an "ignore" strategy, certainly not at this stage; indeed, there may well be grounds for the OP to get the PF cancelled claim a partial refund of the money already paid, being the difference between the correct fare* and the higher fare actually paid).
*Assuming, for the sake of argument, that the PF was incorrectly issued and that the OP was entitled under all the circumstances to pay a lower fare, which is not entirely clear.