I’m going to guess the letter is from Roxburgh debt collecters.
A nasty bunch but harmless.
Regardless of who it is ignore them. NOTHING can happen to you. they have no rights of entry to your house and they have no way of affecting your credit rating.
The ONLY way this can happen is if you are taken to court, and loose, and don’t pay then you’ll get bailiffs and CCJ’s
If they ring make sure you have the company name then tell them “the debt is denied and and any further contact will be considered harassment and will be reported as such” then hang up.. unless you want to have some fun and keep them on hold or something.
For future reference you do NOT have to pay parking charge notices from private parking firms.
I would never suggest you go around dumping your car where you feel. You were paying for a service, however £50 for a 15min overstay is not a reasonable estimate of losses the MOST you owed them was 15 minutes of parking fee.
If it happens again Ignore everything from them (but don’t throw it away)
If you get a council ticket (PENALTY charge notice) you have to pay that or they do get in bailiffs.
Have a look on Pepipoo for reassurance and others experience