It's all very simple.
Pre-Gidden, the law related to the Interpretation Act 1978 s7,which allowed that once a letter had been correctly addressed, prepaid and posted, that it was 'served'.
The Justices in Gidden held that this was not, in fact, the correct way and that 'service' meant an item of post having been received by the intended recipient. This had the effect of making service by post a rebuttable presumption.
Thus, upon the sending of the NIP, it is for the defendant to show on the balance of probabilities that he did not receive it, and so it was not served, within the fourteen whole days after the date of offence.
This is achieved by the serving of a signed s9 Witness Statement to that effect.
The prosecution then must show, beyond all reasonable doubt, by cross examination at Court or by some other compelling evidence, that the content of the s9 is false and that the service of the NIP was valid.
So, in this case, mummy dearest serves upon the CPS the aforementioned s9, the Crown Prosecutor decides that he has bigger fish to fry than a petty speeding matter, and duly drops the case like a hot potato. You get a NOD in the post, job done.
SO Simon, I was flashed on the 20th November in the Limehouse tunnel, it is now the 5th December, thats 15 days after the event, should the NIP arrive now I can return it with an accompanying letter of "late service" or should I get the postman to witness the opening of the mail and sign accordingly?.