The payment period in the PCN is:
The penalty charge mist be paid within the period of 28 days beginning with the date on which the penalty charge notice is served. You must either:
'Pay the penalty charge before the end of the period of 28 days beginning with the date of the notice...
The Act says:
iii)that the penalty charge must be paid before the end of the period of 28 days beginning with the date of the notice;
So, the PCN does state what's required. It also gives a form which is not correct. So, OP let's be clear. The statutory periods are wrong is as wrong as the error being described. It's too far reaching. You need to be more focused IMO.
The PCN gives 2 payment periods, one (the first) beginning on the date of service and the second 'beginning on the date of the notice'.
Self-evidently there can only be one, and given that the practical effect of the difference is a minimum of two days and a possible maximum of 4 days means that the PCN fails to meet the standard required by the Act.