The letter you have chopped is from DCB Legal, not from the allied debt collectors DCBL.
Regardless, you now wait for a letter of claim and file a defence of the “double dip” that the parking company missed, which you could have done at the start.
You can also reply to DCB Legal to the same effect now, but they will probably say it’s too late to appeal now, it won’t matter since you’re mainly doing it for the record.
Whatever the response, the claim will eventually be withdrawn provided you stay on top of the process.
In the meantime, DCB Legal will try and scare you into paying up, including increasingly desperate attempts to “settle” in due course, all of which you should ignore.