Although photos are not a legal requirement, most councils get their CEOs to take them to back up the evidence in the CEO's notes. So you should look in their website quoting the PCN Number to see if there are any. I would expect to see a photo of the yellow envelope containing the PCN on the car windscreen.
By not finding the PCN on the car, you have, essentially, been denied the opportunity to pay the discounted amount, so any reps you send in should mention this. CP8759 may come up with something else. If their response reinstates the discount you have a decision to take; pay-up at the discount, or risk the full PC penalty at the adjudicators.
BTW, the restriction used to end at 6.30 pm. See GSV view of November 2020
https://maps.app.goo.gl/58tiDaUSsja29fZ69Did you used to park there then, and just assumed the times were the same today ?