Not sure which is true.
You Must receive the nip within 14 days
or
They have to issue the nip within 14 days.
It may be possible that someone doesn't receive it until after 14 days of the (alleged) offence.. If the police say they issued it within the 14 days does that mean you can be prosecuted? .. i.e. does their word stand in court against yours?
As they send nip's just normal 1st class mail, and not "signed for" (recorded, registered mail, etc) they cannot prove you received it.