You appear to have a few days remaining in which to make an appeal - but on what basis? The argument will be that the driver read the signs and parked, entering into a contract, and is now being pursued for breach of contract on some basis, presumably because of a parking time limit. The registered keeper can be made liable under the Protection of Freedoms Act 2012 in place of the unknown driver, but that’s why the original notice may be important to see.
That said, I think you should direct your efforts first to the owner of the location to see if you can get them to cancel this for you.
