So it looks from your photos that you were parked here: -
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and there
is a sign on the side that you parked, but the cretins have put it by the bay adjacent to the gate across the street.
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This bay is separated from yours by the double-yellow lines. This means the bay you parked in is in a separate set of bays that require their own sign, but as you say, there is no sign.
So you should submit representations stating that the bay you parked in was one of a group of (Nn), but had no sign with the bay restrictions. With no sign, this means the bays are uncontrolled and the PCN is therefore void, so you request it is cancelled forthwith.
The problem you will have is that Redbridge, being greedy for your money, will just send you a Fob-Off letter, thus forcing you to either pay the discount, or wait for the Notice to Owner and submit a formal representation this time. The discount is lost at the NtO stage, although it is likely Redbridge would re-offer it when they reject your formal reps. Really the only way to get a result is to take them to London Tribunals and risk the full PCN penalty. Councils know people are reluctant to do this, so they ruthlessly game the system to "encourage" payment.
So are you the owner with your name and address on the V5C for the car, and is the V5C address up-to-date ?