The only reason to delay appealing to day 26 is if you received an NtD, which you didn't you received an NtK. As long as you appeal on or before 29th May, you will be OK.
You appeal only as the keeper. Of course you mention the breach of PoFA 9(2)(a) and (b). It is that failure to comply with the strict requirements of PoFA that means they cannot transfer liability to the keeper. They have no idea who the driver is. Do you see where this is going?
PoFA 9(2)(a) and (b) state:
A notice which is to be relied on as a notice to keeper for the purposes of paragraph 6(1)(b) is given in accordance with this paragraph if the following requirements are met. The notice must specify the vehicle, the relevant land on which it was parked and the period of parking to which the notice relates; inform the keeper that the driver is required to pay parking charges in respect of the specified period of parking and that the parking charges have not been paid in full.
Please show where the "specified period of parking" is mentioned anywhere in the NtK. By failing to comply with the PoFA requirements, they can only pursue the driver. They have no idea who that is.
By failing to fully comply with the strict requirements of PoFA, they have also breached their own ATAs CoP at 2.4, 19.4, 21.2, 21.12 and 21.13 to name a few.
I would also query the fact that the vehicle is not parked on the car park. It is parked on the pedestrian area. I would demand that they provide evidence of their contract with the landowner that proves they are allowed to issue PCNs to any vehicle that is not parked in the actual car park.