Their letter of rejection is dated 12th August, but you've only posted it here on the 21st August. It says you can pay the discounted amount, (£65), within 14 days of service of the letter.
So when did you receive the letter, because by my reckoning, you are coming up to the end of the 14 day period soon ? Reading the thread, no strong appeal argument seems to have been suggested, so it is your decision; pay the discount, or register an appeal at London Tribunals with the full PCN penalty in play.