For heavens sake... stop going on about useless "debt collectors". You must be low-hanging fruit on the gullible tree if you think that a debt collector can do anything about this alleged debt. They can't. All they can do is try and get you to poop your pants and pay up out of ignorance and fear.
The debt collector is not a party to the contract allegedly breached by the driver and there is absolutely nothing they can do that can affect you. Ignore any debt collector if you ever get one.
That Parking Charge Notice (PCN), issued as a postal Notice to Keeper (NtK) cannot hold the Keeper liable because it is not compliant with all the requirements of PoFA. They have no idea who the driver is unless you blab it to them, inadvertently or otherwise. There is no legal obligation on the Keeper to identify the driver to an unregulated private parking company and you should not bab the drivers identity by saying stupid things like "I did this or that". You are the Keeper and you always refer to the driver in the third person such as "The driver did this or that". Do you understand so far?
I have no idea what "reply" you are referring to but I suggest you send the following, only as the Keeper, as your appeal to this Mickey Mouse bunch of ex-clampers:
Easy one to defeat... as long as the unknown drivers identity is not revealed. There is no legal obligation on the known keeper (the recipient of the Notice to Keeper (NtK)) to reveal the identity of the unknown driver and no inference or assumptions can be made.
The NtK is not compliant with all the requirements of PoFA which means that if the unknown driver is not identified, they cannot transfer liability for the charge from the unknown driver to the known keeper.
Use the following as your appeal. No need to embellish or remove anything from it:
I am the keeper of the vehicle and I dispute your 'parking charge'. I deny any liability or contractual agreement and I will be making a complaint about your predatory conduct to your client landowner.
As your Notice to Keeper (NtK) does not fully comply with ALL the requirements of PoFA 2012, you are unable to hold the keeper of the vehicle liable for the charge. Partial or even substantial compliance is not sufficient. There will be no admission as to who was driving and no inference or assumptions can be drawn. UPE has relied on contract law allegations of breach against the driver only.
The registered keeper cannot be presumed or inferred to have been the driver, nor pursued under some twisted interpretation of the law of agency. Your NtK can only hold the driver liable. UOE have no hope at IAS, so you are urged to save us both a complete waste of time and cancel the PCN.
Don't be surprised though if they reject the appeal. They are scammers and they are hoping you are stupid enough to not realise and pay up. An IAS appeal would be futile but you may as well try one if they reject the initial appeal.
As long as you follow our advice and don't overthink this and stop panicking about useless debt collector letters, you won't be paying a penny to these ex-clampers.