You say you informed DVLA of the change of address - can you prove it. For example have you any correspondence or acknowledgement from them?
It's safe to assume that the NIP was sent out in good time, but you would have to be able to demonstrate that is was not your fault.
A notice shall be deemed to have been served on a person if it was posted to him at his last known address, notwithstanding that the notice was returned as undelivered or was for any other reason not received by him. The posted NIP is deemed to be served until the contrary is shown.
How did the NIP reach you - was it redirected by someone at your previous address or sent directly by the police?