What have Saisbury's said about this? How far up the management food chain did you go when enquiring? Did you try emailing the CEO with the details in order to ask them to get this cancelled?
You should send a complaint (not an appeal) to ECP and inform them that their original NtK was not served to the correct address and they should follow the the procedure as outlined in the
BPA CoP:
23.8 You must have a process for considering appeals received outside of the normal 28-day period allowed for lodging an appeal where the appellant provides evidence of exceptional circumstances for the appeal not being lodged within the normal timeframes – where the addressee only discovers and can show that a parking charge notice has been issued in their name after the 28- day period the period must restart and any enforcement, excluding court action, must be paused.
They should in this instance, pause all enforcement and accept an appeal. Failure to do this will result in a complaint to the BPA.
Whether you failed to update your V5C is neither here nor there. It is up to the DVLA if they want to prosecute you for that fairly common omission. An unregulated private parking company has no authority to penalise you for that.
You are able to show that you only discovered that a PCN had been issued in the keepers name after the 28 day appeal period had expired.