Do you have evidence that you were “working” as a delivery driver and that you were simply loading and unloading at the time? If you do, then this is my advice:
Loading and unloading in the course of your work is not parking and is clarified in persuasive case law. However, that is not going to help you unless this goes to a claim, which it will.
No appeal to PCM or the IAS is going to get this cancelled although you are free to try and appeal on that basis. Personally, I would completely ignore everything from this firm of ex-clampers until a Letter of Claim (LoC) is issued.
An LoC can be responded to and a claim easily defended. Everything before that is going to be a wasted effort although others on here would advise you to go through the motions of the useless appeals process. To be honest, nothing before the LoC really matters and would not affect any claim if there was no response before litigation commenced. It’s your time that is required for a well trodden path of wasted effort.
Keep evidence of the job. Ignore all debt recovery threats and come back when you receive the LoC for advice on how to respond. Others may advise differently, but the eventual outcome will be identical.