No you haven't missed anything, it is silly me for missing it ! Obviously the lateness of the hour caused me to miss the obvious ! However you didn't post your reps, so maybe that's why.
This looks like the Fob-Off letter to beat all others, where not only have they have totally ignored your representations, they haven't even bothered to look at the sign shown in their own letter of rejection, and compared it to the PCN time to the sign !
Under the enforcement process, what you have to do now is wait for the Notice to Owner and submit a formal representation, but no harm will be done by sending a second challenge to them asking them to compare the times on the sign, with the time on your PCN.
If they resist all the way to London Tribunals, you'd be justified in seeking a costs order.