Have you still got the parking ticket that blew down onto the seat ? It is very unwise to leave old tickets in the car as you did, because you end up with the circumstance you find yourself in. So if you have the original ticket, take a copy of it and include it with your reps. Best also not to be confrontational in any way.
You have the absolute right in law to take the matter all the way to adjudication, but to get there you have to go through the process.
If an informal challenge to the PCN is rejected, you can wait for the Notice to Owner and submit representations again. Most informal challenges are ruthlessly rejected, because the council, eager to get the money, just refuse them knowing most people then pay. Are you the owner of the car, and is the V5C Registration Certificate up-to-date ? The NtO is sent to the V5C address
The discount is lost at the NtO stage, but if they refuse reps against the NtO , it's then a total no-brainer to take them to adjudication, because the penalty does not increase, and there are no additional costs. If you win you pay nothing, if you lose, it is the full PCN penalty to pay.