Date of postmark is considered applicable date, not date of receipt.
If said date is out with 14 days of date of offence then defence is complete and case is liable to be binned.
I agree with this from my own personal experience, whether I was conned or not is another matter.
I have posted this on here several times before.
A few years back I got done for speeding by a camera van. My NIP arrived 31 days after the offence. I was driving my own car, registered to me for at least five years and I had not changed address for at least 20 years. I had heard about the 14 day 'rule' so I challenged it, admittedly only by post. I received a letter back stating that ' The NIP had been sent well within the 14 day deadline but had been delayed by a postal strike. The intention was for it to arrive within the 14 days and it was reasonable assumption on their part that it would. They had no reason to think otherwise'.
As I have access to a legal team through my company, I showed them the letter with a view to challenging it on principle rather than reality, as I was actually guilty. They advised me that the NIP still stood as it had been sent within the time frame and they had every belief that it should arrive within that time frame.
Like I say, maybe I was conned, but as I was guilty I took the hit.