You are talking about the "mugs discount". It is designed to get "mugs" to simply make their lives easier by conning the gullible into paying them without having to go through the process of appealing or litigating.
It's your choice. If you think you should simply pay an unregulated private parking company £60 because they have sent you a speculative invoice, then go ahead. If you believe that you owe a bunch of ex-clamper thugs £60 because they simply say you owe them, that is up to you.
THese companies have a modus operandi. They issue these "invoices" in the hope that the recipient is low-hanging fruit on the gullible tree, ripe for the picking. They know that the majority of their victims are ignorant of the law and their rights.
As I already mentioned, no one who is here receiving advice, pays a penny to ECP. If you feel £60 is worth your while to fund the scammers, it's up to you.