Having examined the parking site in some detail, it is highly likely that the operator is simply using the 'law of averages' to make a good return - meaning that enough people pay up out of ignorance or fear.
Given the site layout, signage and very short consideration periods demonstrated on the PCNs, the operator probably knows full well that they would have very little chance if the matter went to hearing. As a result, they just play the game of statistical chess which gives them the best results without ever needing to set foot inside a courtroom.
As others have said, keep all evidence safe. Also be careful if you move house in the next few years because these firms do try and resurrect older cases - You don't want paperwork going to an old address as this can lead to a default judgement.