Not only have you not received the PCN, but also the Charge Certificate that increases the penalty by 50%. Have you had any mail problems at all ? Is the address on your V5C for your car correct in all detail, up-to-date, and the same as on the contravention date ?
As you are still well in-time to submit a Statutory Declaration, (Form PE3), you should use the form included with their OfR, and submit it to TEC. It is best to send it via email. I'm not sure of the detail, but somebody will on here will tell you the process. Once your SD is accepted, the matter reverts back to the original PCN, at which point you can submit representations against it, or pay it.
When filling in the form, the "applicant" (see top right of form), is the council, not you. You are the "respondent"