If you pay this, you are a fool and your money! No one pays a penny to ECP if they follow the advice, even if they blabbed the drivers identity in their appeal.
If all you are interested in is the mugs discount, then you are a lost cause. Paying a speculative invoice from an unregulated private parking firm just because they make an offer of a discount is ridiculous.
Of course you appeal to POPLA, for what it's worth. You may get lucky. Depending on whether the driver has been identified or not, you can search the forum for other POPLA appeals and give one a go. However, in most cases with ECP, you will not be successful at POPLA.
A POPLA decision is not binding on the appellant. Where this is won is after a county courts claim has been issued and defended. The claim will, 99.9% for sure, be struck out or discontinued before they have to pay the £27 trial fee.
