SNAFU. How low does the fruit hang on the gullible tree? "Enforcement", "Officer", they really have managed to overwhelm you with their scam.
What exactly did you put in your appeal? Did you blab that you, the Keeper, was also the driver? Have you admitted liability for the alleged debt because you paid the speculative invoice that you imagined was a "fine" from an unregulated private parking company?
Never, ever, ever, EVER, communicate with a useless and powerless debt collector, especially one as bottom-dwelling as ZZPS and their sister company GCTT. No debt collector or Debt Recovery Agent (DRA) is a party to the contract that the driver allegedly breached with NPM. Use their letters as toilet paper. Better still, just ignore them.
Your Parking Charge Notice (PCN) that was issued as a postal Notice to Keeper (NtK) is nothing more than a speculative invoice. Would you just say any speculative invoice silly because you are offered a 40% discount? Do you beige that the PCN was issued fairly and the driver owes NPM some money because they claim the driver breached a contractual term on a sign?
In order to assist you and try and undo some of the mistakes already made, follow the advice given above and show us the NtK and what exactly you put in your appeal. Also confirm whether you made an actual payment that was accepted and debited from your account and exactly who was this payment made out to.