Then you may have the makings of a successful defence on the basis that no NIP, compliant with s1 of the Road Traffic Offenders’ Act, was served.
To take advantage of this you will have to turn down (or ignore) any out-of-court offers the police may make and instead face prosecution in court. The police have only to prove to the court’s satisfaction that the NIP was posted to the Registered Keeper’s last known address in time so as to arrive within 14 days. If they do that, it is deemed “served” two working days after posting.
The burden then shifts to you to convince the court “on the balance of probabilities” (i.e. more likely than not) that it did not arrive in time. Of course if the police can only prove that it was posted on 18th August, that burden will not fall on you.
However, I have a suspicion that you may have got something wrong somewhere. I say this because we understand (though cannot be sure) that the system which produces NIPs will not automatically produce one if it will not meet the 14 day deadline. Manual intervention is then required. As I said, this is only what we understand to be the case.
You need to check everything very carefully before embarking on this course because the cost of failure in court is high. You will pay a fine of half a week’s net income, a “victim surcharge” of 40% of that fine and prosecution costs of around £650. For most people that will mean a total of at least £1,000 and probably more. Before you make your decision, it may be worth asking the police whether the NIP you have, dated 18th August, was in fact the first NIP they sent.
As an aside, were you not offered an awareness course for either of these offences?