We've seen this a few times and even going back to the old Pepipoo forum ! Signage is key to success with an appeal here, and by 'appeal', I mean London Tribunals. London councils reject virtually all reps because they know most people then just cough-up, (>95%). So you must steel yourself to stand your ground and put all thoughts of the discount out of your head.
It may be worth looking at cases at London Tribunals using the location on the PCN as a search argument, because there have been many wins here. Having said that the council may have adjusted the signage since the last successful case, (but I doubt it)