Not quite so, northstar.
The legislation says that the NIP must be �...posted so that in normal circumstances it will arrive [at the registered keeper�s address] within 14 days�. �Delivery�, �receipt� or �service� is not mentioned as the notice is considered to have been served provided it has been posted in time.
If a NIP is received outside 14 days the driver can opt to refuse a fixed penalty offer and appear on a speeding (or failure to provide drivers� details) charge in the magistrates� court. If he can show that the NIP was delivered late (and the onus is on him to show that it was, not the prosecutor to show that it was not) he can go on to argue that the late delivery of the NIP unfairly prejudiced his right to a fair hearing because (for example) too much time had elapsed for him to remember the details of the alleged offence.
It will then be for the justices to decide, given all the circumstances, whether this is so and whether to allow the prosecution to proceed.
In short, late delivery of a NIP (because, say, it was delayed in the post) does not automatically invalidate the proceedings. NIPs very often arrive with the driver (who ultimately is the person to face a speeding charge) outside 14 days when he is not the Registered Keeper. There are also other valid reasons why a NIP arrived outside 14 days and in then it becomes a decision for the magistrates to say whether the reasons for late delivery are sufficient to allow the matter to proceed.