Just follow to the letter what is advised here, and you should be good. I am currently in a dispute with the same UKCPS for exactly the same issue of 'stopping'. In fact, I honestly thought I was reading my own thread when I read this.
I have been doing a lot of reading up on this since my case started, and it is evident how corrupt these companies are, and as b789 will point out, these companies survive on scaring people into paying. The majority of whom will pay up once faced with the threat of court, CCJs and bailiffs, which are only enforceable once the court has been involved, sent you a letter, you haven't responded or paid – that's when that will kick in, however, because of how these parasites operate, and because people pay up, they rarely get that far.
From what I have found, irrespective of the time frame of when the letter is sent out to when you actually receive it, you mention your letter as "PCN" – where does it point out that this is a PCN? At best, this is an invoice and nothing more, but even that is debatable.
If you look up on the .GOV own website who can legally issue a PCN, and you'll find that a private company can't issue one for 'stopping', only violating parking within a private car park. As such, and because you haven't signed anything, this holds no 'legal' standing so no parking company, debt collection, or even legal team can make threats of issuing CCJs, they can only apply for this to go to a magistrate's court to recoup their losses of you allegedly breaching the T&Ts they claim you have accepted by entering the area they allegedly control.
DO NOT CONTACT ANY OF THESE COMPANIES DIRECTLY BY PHONE OR EMAIL, other than as instructed here.