A Scottish Keeper cannot use POPLA unless they are appealing as the driver. @jfollows, when making definitive statements, pease make sure you are not misunderstanding the legal implications of your statements.
For the OP, Do not even attempt to make a POPLA appeal. Do not contact the parking company again.
You haven't shown us exactly what you put in your appeal but as long as the driver has not been identified, just get on with your life and ignore everything that these rogue companies send you from now on. NOTHING is going to happen.
Ignore all useless debt recovery letters. These bottom-dwelling firms are powerless to do anything except try and scare the low-hanging fruit on the gullible tree into paying up out of ignorance and fear. Never, ever communicate with a powerless debt collector. You can safely shred their letters and use them as hamster bedding. Ignore.
Eventually, they will give up because they have absolutely no way yo recover anything from the Keeper and the have no idea of the drivers identity. Asking them to help you identify the driver is a really silly move. Why on earth would you want to assist an unregulated private parking company to identify the driver... in Scotland?
For now, in Scotland, if you receive a Notice to Keeper (NtK) for an alleged parking contravention on private land, you simply appeal as the Keeper and refer them to the answer given in Arkell v Pressdram (1971). Nothing will happen.