There are no local signs, so one must assume you are in a Controlled Parking Zone (CPZ).
Is it possible to post up your representations, please. The rejection is their usual Fob-Off letter, which I assume has re-offered the discount. London councils refuse almost all informal challenges, because they know that people then just cough-up to get the discount.
It is obvious that the yellow line is worn even without builders dust covering it, but if you want to fight these PCNs the council, (who keep all the money if you cough-up), will refuse all reps until you register an appeal at London Tribunals when they might give way, but the discount option will be long gone by them.
FWIW, I think you have a reasonable case, but it is certainly not a "slam-dunk" win, in my opinion.