"Acceptable" is not a concept known to road traffic law.
There is only one 14 day rule. Unless you can prove otherwise, a NIP will have been deemed to have been served on the Registered Keeper within the 14 days. If it was delivered to his last known address, then it was lawfully served on him.
If the requirement had not been satisfied, and if you could somehow prove that, then the next question would seem to be whether you contributed to the failure - in which case the requirement to serve a NIP would not apply at all.
TL;DR - suck it up, buttercup