Your NIP is probably effective !
You have confirmed that you are the registered keeper of the car and that you did not receive the NIP within 14 days. You have also confirmed that one of the following applies to you;
The legal burden on the police is to send the NIP to the name and address of the person as recorded on the DVLA database. If the DVLA do not have the correct or up to date details then the police will send the NIP to the wrong address/person – but they will still comply with the requirements of s.1 RTOA 1988. The police will simply argue that they did send the NIP to the last known address of the registered keeper. As long as they have sent the NIP to the person/address on the DVLA database – they will have complied.
If you know who was driving then you have to name the driver – a defective NIP is not a defence to failing to name the driver.
If you don’t know who was driving then you may have a defence to failing to name the driver and the fact that the police took a while to tell you about the offence may help your argument. You need to get in touch and tell us what has happened.