Hi.
The system works like this:
When a PCN is issued, the Local Authority will issue a Notice to Owner to the person they believe to be the owner of the car. Usually they just find the registered keeper of that registration number, and they "believe" them to be the owner of the car. You instantly become legally liable and get an NTO (Notice to Owner) and have 28 days to contest liability.
In this case, the NTO was dated 23 December and you did not contest it in time. Not your fault perhaps, but nonetheless you forfeit your right of appeal.
Now for a possible route you can take...
If you were genuinely unable to contest the NTO because you were abroad, then you can ignore the charge certificate and wait until an Order for Recovery is sent to you.
At this point you will have the right to file a Statutory Declaration which is a legal challenge to the Charge Certificate. It's legal, but not too complicated to do. The letters will explain how. You will file it on the grounds that you did not receive the Notice to Owner - you were out the country.
NOTE - it is NOT a challenge to the original PCN, just to the charge certificate. Normally, if you do this in time (as stated on the document) it will be accepted without question.
What it does is compel the local authority to cancel the charge certificate and issue you with a new Notice to Owner.
Assuming it goes as it should, then "bingo!"
Now you have 28 days again to contest liability for the original PCN, on the grounds that it was a cloned plate. Send them a photocopy of your tax disc at the time, as the parking attendants normally record the serial number from the disc when they issue a PCN. Obviously if it wasn't your car, it will have a different tax disc in the windscreen.
If this isn't viable for some reason it's more tricky but comparing photos will probably do - they will look for clues on their photos taken when the PCN was issued - such as the styling of the number plates, upholstery (if visible), chips or dents in the paintwork, they types of bumpers, and so on to verify whether it is a different vehicle. They should be happy to send you copies of their photos if you ask, so you can see for yourself.
It is also important that you do not try and enter into an exchange of correspondence with them. They won't play ball and it screws up the process and works against you! Wait for the new Notice to Owner then send everything - your representation, copy of the tax disc and some photos of the car all at once. The whole bundle will constitute your appeal. If you leave some out, say the tax disc details, they will process your first letter without it and your sole chance to submit a representation will be 'used up'.
Also - speak to DVLA and the police about the cloning... more serious things than parking offences could come your way, so get it on record that someone's cloned your plates!
Hope this helps.