28 days from date of offence is irrelevant.
27 days from deemed date of service of the NIP would be relevant if they hadn't issued a reminder. As long as you satisfy the requirements of the reminder, all is good as regards the s. 172 requirement.
The way to tell whether your details on the V5C are correct and current is to hold the V5C in your hand and check them. Telling us that they are because some other paperwork found its way to you tells us more about you than the V5C.
If you are the RK and the details on the V5C are correct, then if no NIP was served (delivered) within 14 days of the date of the offence, then that is a defence to the speeding allegation - as long as you can persuade the court with credible testimony that, on the balance of probabilities, no such notice was delivered to your address.
