The first appeal which should be called a "challenge" is required if you wish to make a further appeal to an independent assessor* at POPLA (Parking on Private Land Appeals). This organisation is funded by British Parking Association members at NO cost to the driver or keeper - so challenges and the subsequent appeals are free to you.
*Legally trained staff have been replaced by "customer service" staff last year so it is questionable whether POPLA is now fit for purpose. Suggest if you have been given a questionable decision by POPLA, that you write to your MP about it.
A list of current BPA Members can be found here.