It is not argued that the parking is for Nandos customers. My core argument, however is:
- given that Nandos was shut at the time I pulled up, surely the parking restrictions do not apply.
As a general point, that's not a sound conclusion. Looked at the other way, parking is for Nandos customers - if Nandos is closed, how can the driver have been a customer? There may be specific arguments about impossibility of performance etc. that you can use in this case, but as a general concept, a business being closed does not confer any automatic right to park on their premises without regard for any terms or conditions applicable to the use of their car park.
As for whether it's "worth the fight" - that's your decision to make. In my previous reply I advised that they're likely to mess up the process of transferring liability to you as the hirer, which is something you can use to appeal. NPE will reject your appeal, because they reject almost every appeal. The IAS are also likely to reject your appeal, but neither of these things mean that you are liable for the charge as the hirer. Whether you're prepared to put in the effort to fight them all the way is for you to decide - it's your money and time.