The Letter of Claim will be from DCB Legal, not from DCBL. Replying to it shows that you’re engaged and is part of a paper trail which can subsequently show the court that you’re engaged and that you dispute the claim. But it’s not major to have ignored it.
You will get a county court claim, with woeful Particulars of Claim, in due course. There are strict deadlines for responding so be sure that you won’t miss this, because you’ll get a judgement in default against you if you do.
The original Notice to Keeper plus your own story behind this will form the basis for your defence, so we need to see and hear these to provide advice. If you don’t have the NtK, get hold of it as a matter of urgency through a Subject Access Request to the parking company (not from DCBL or DCB Legal).
Presumably you also ignored the NtK, meaning that you have not identified the driver to the parking company? This may or may not be important for any defence.
The claim will be based on the driver entering into a contract communicated by clear signs at the time, and transferring the liability to the registered keeper using the provisions of the Protections of Freedoms Act 2012.
Properly defended claims are usually discontinued before having to pay the court fee by DCB Legal.