If the PCN is issued as a windscreen Notice to Driver (NtD) then PoFA paragraph 7 applies and the 28 day response period you are referring to is is if the NtD is not responded to and a follow up, postal Notice to Keeper (NtK) is then issued.
There is no requirement to issue an NtD, whether ANPR or a patrol takes the evidential photos. If no NtD is issued, then a postal NtK can be issued and the "relevant period" for the 'service' of that, in order to be able to transfer liability from the unknown driver to the known Keeper, is 14 days from the day after the date of the alleged contravention.