Ignore anything from DCBL or any other debt collector. They are powerless to do anything except to try and persuade the low-hanging fruit on the gullible tree to pay up out of ignorance and fear.
Never, ever, EVER enter into communication with a useless debt collector. You have already done that and you are marked by them as low-hanging fruit.
If you follow the advice you now receive, you won't be paying a penny to (not so) Smart Parking. Continue to ignore any dent recovery letters.
Eventually, you will receive a Letter of Claim (LoC), most likely from DCB Legal (not DCBL) and you should let us know. We will provide a suitable response.
In due course, you will receive an N1SDT Claim Form from the CNBC. Again, show it to us and we will advise on how to respond and provide a template defence.
After many months, the claim will either be struck out or they will discontinue and that will be the end of the matter.
The "parking code of conduct" [sic], whatever that is, does not say that "customers" must be given a grace period on entry and exit. What the Code of Practice (depending on whether the operator is a BPA or IPC AOS member) allows is a minimum of 5 minutes "consideration period" which is not added to any allowed parking time. Only if a contract is established with the driver and, in this case, a parking session is purchased, then there is a "grace period", usually a minimum of 10 minutes beyond the purchased parking period.
So, you have done everything wrong up to this point, based on incorrect assumptions. However, as I have said, if you follow the advice from here on in, you won[t be paying a penny to (not so) Smart Parking.