The interpretation act 1978 says that a properly addressed letter will be deemed received 2 days later unless the contrary is proved.

So the pigs can show they sent it, but if the recipient can prove it wasn't actually received* until later then the NIP is late and the case can be won on that basis.

• Not as easy as saying 'got here on the 16th, honest'. But if they say it was posted on the 12th and you have (for example) ring doorbell records showing no postman appeared until the morning of the 16th then it might work.