As far as I can see the PCN successfully deals with a quirk in the controlling Act, (the LLA & TfL Act 2003), which have different periods for payment and submission of reps. Its the same 28 days, but payment period starts from PCN date, and reps submission starts from date of service, 2 days later.
There may be a technical argument based on their website pages, but this sort of thing is invariably only dealt with at London Tribunals, and you have to risk the full PCN penalty, the discount option is lost.