Best if you post-up whatever documents you have. If the PCN number is on the CC, use it and your reg number to look at the PCN and evidence on the council website and post that as well.
Next stage of the process, if you don't pay the CC, is for them to register the debt at the Traffic Enforcement Centre, (the "court"), and send you an Order for Recovery. This adds £9 onto the CC amount. At this point you can submit your statutory declaration. Look here for the forms: -
Traffic Enforcement Centre forms, including the form to challenge an unpaid penalty charge notice.
Until you tell us what the PCN is for, we can't tell you what form to use. Provided you submit the declaration within the time period allowed after issue of the Order for Recovery, it will be accepted and the matter reverted to the PCN stage. You cannot submit any declaration to TEC until the PCN is registered there.