If you do not have the original PCN then you can send a Subject Access Request to the parking company to obtain this, which you should do as a matter of urgency.
Did you also send a SAR to CEL (Civil Enforcement Limited)? If not, DCBL is not the parking company, so won't have sent you anything useful, DCBL are debt collectors.
Search the forum for "mediation" (not “medication”) but essentially you offer £0 to settle and it won't be accepted. End of mediation session.