How often do you collect your mail ? A PCN date of 12th Nov, would be expected to be served 2 days later as it is sent (must be, was it ?) by 1st Class mail. You then have 28 days to pay or submit reps. I suggest you submit reps on he basis that the PCN arrived in your mail on <date> so was well outside the 28 day period, and ask them to cancel the CC and accept late representations.
If they refuse all this, you can wait for the Order for Recovery, which is sent following then registering the debt at the Traffic Enforcement Centre. This adds £10 to the CC amount, being the TEC registration fee. You can then submit a Statutory Declaration and state with a clear conscience that you didn't receive the PCN. Yes, you got it but far too late to do anything about it, so I am quite certain your SD reason is valid. Provided it is submitted in time, your SD will be accepted, and the matter reverted to the PCN stage when you can pay the discount or submit reps.