BW legal 4 years on

[Mcwinfrey]

Hi and thank you in advance for any help. Four years ago my wife was leaving a car park and typically overstayed her time by chatting to a friend who happened to be parked next to her and also about to leave. The lovely parking chap walked up and stuck on her windscreen pcn in front of her as her friend quickly drove off escaping the ticket. Shortly after we received a letter with photo of the car and one driving off from the private company which, right or wrong, we ignored. We then ignored another couple including one from a no win no fee bunch of rogues who then passed it back to the original form for one last attempt before going quiet. Four years later, a week after throwing away all of the letters in a drawer tidy out, we receive a letter from BW legal requesting the money plus their fee. The details of the car park, registration number and offence are at the top of the letter but no evidence is supplied. The letter threatens court. What is the best approach please? Thanks again