If you want to get your parking ticket cancelled you would do well to read the following cancellation
policy procedure and try and fit your representation or appeal (the first informal letter is a representation and the second official one is an appeal apparently - Mr Mustard expects that the distinction is lost on most people) into one of the categories and provide the listed supporting paperwork so that the ticket should be cancelled.
PCN Cancellation Procedure v1.15 1 April 2010
Even if your appeal is on different grounds that are not listed it is still worth sending it in as it might be accepted and the penalty is frozen at the 50% payable level until your representation is answered.
Do follow the instructions on your parking ticket and appeal either using the form on the council website or in writing as instructed on the parking ticket. Don't leave the matter as it only gets more expensive if you do nothing.
Don't worry if your informal representations are rejected. Simply re-submit the same grounds at the formal stage. Mr Mustard suspects that a lot of bluffing goes on and that if you take your appeal all the way to PATAS (Parking & Traffic Appeals Service) a lot of cases get dropped by the council at the last minute.
It is a good idea to download the document to your computer, print it out and give it to friends who have had a parking ticket and/or email it to everyone in your address book. It might be the next present you have ever given them?