The signs in your GSV link for Castlewood Road are the usual 'Flying Motorbike' signs used by councils to bar motor traffic at certain times, and as a motorist, you are supposed to know what they mean. The GSV view is June 2022, so they have been there for over 3 and a 1/2 years, so not considered new now.
PCN is timed at 15.36, so well into the afternoon closed period, which is actually quite short, we normally see much longer times.
Don't just cough-up, but submit representations on the lines you have put here, with the emphasis on not being locals, but if you live somewhere else in London this won't be a strong point at all as these closed streets are all over London now.
Another point to make is that it is a left turn into Castlewood Road, and there is no advance sign to warn an approaching motorist, so put that in your reps and say that the restriction came as a complete surprise.
You may get luckly, but if not, they will re-offer the discount.
