I've had a few from the companies that operate on supermarket car parks etc, ignored them all and nowts come of it. In those cases the "PCN" is merely an invoice stating you entered into a contract with them by parking in their car park and owe them money for parking there. However despite the rule changes to who they can and can't pursue, nothing has changed on the countries contract law. AFAIK it still requires the claimant to prove you knowingly entered into broke their contract by parking there, AND that the cost they're claiming back from you is fair and just.
As for this one parking on what seems to be a roadside, no idea never really heard that before. Are there clear signs stating it's someone elses bay and if so you can't park there? If not, I can't see how anyone would prove you knowingly breached contract. The whole thing of the ticket being incorrect so not standing as often works for official police/council tickets won't work here in the same way I wouldn't have thought, but would stand you in good stead to argue you parking there hasn't breached it, and would certainly limit their ability to prove what they're claiming from you is fair, if not it becomes a "penalty" by legal definition which I believe can't legally be levied by a private company under contract law.
What company was it, and how much are they trying to ask for?