If you have evidence to support your claims such as receipts, photographs, or witness statements, send these in. The council will write back, either accepting or rejecting your challenge. If you have written within 14 days of receiving the PCN and the council rejects your challenge, you should be offered the chance to pay at the discounted rate.

The Law - A ticket is valid only if it meets the requirements below
http://www.opsi.gov.uk/acts/acts1991/Ukpga_19910040_en_3.htm#mdiv66

Parking penalties in London.
 
  66.—(1) Where, in the case of a stationary vehicle in a designated parking place, a parking attendant  has reason to believe that a penalty charge is payable with respect to the vehicle, he may
 
 (a) fix a penalty charge notice to the vehicle

 (b) give such a notice to the person appearing to him to be in charge of the vehicle.

 (3) A penalty charge notice must state

 (a) the grounds on which the parking attendant believes that a penalty charge is payable with respect to the vehicle

(b) the amount of the penalty charge which is payable

 (c) that the penalty charge must be paid before the end of the period of 28 days beginning with the date of the notice;

(d) that if the penalty charge is paid before the end of the period of 14 days beginning with the date of the notice, the amount of the penalty charge will be reduced by the specified proportion;

(e) that, if the penalty charge is not paid before the end of the 28 day period, a notice to owner may be served by the London authority on the person appearing to them to be the owner of the vehicle;

(f) the address to which payment of the penalty charge must be sent.


visit :parking-appeals.gov.uk for more infomation