You can see the sign for the bay on the lamppost near the bus stop. There is a repeater sign about the RPX, but it's on the other side of the zebra crossing: -
https://maps.app.goo.gl/TKXvUZMBGPKqrbfx6Essentially, whilst you may struggle to overturn the PCN it is worth submitting representations that whilst you were outside the bay, you thought you had parked legally, but have subsequently been shown the RPX repeater sign. However, this sign is situated on the other side of the Zebra crossing, and you consider that a repeater sign on the side where you parked would have been noticed by yourself; you saw no reason to look for a sign other than the sign for the bay which is for Blue Badge holders, so you parked outside that in what was clearly a parking bay. as it was set back from the main carriageway.
Post up their response when you get it, but don't get upset if they reject it, as almost all informal reps get a Fob-Off letter in response thus daring you to risk the full penalty.