As you now understand, the NIP is irrelevant. Not least because only one is required - that to the Registered Keeper. The police are not obliged to provide you with a NIP at all.
The relevant document is the "Request for Driver's Details". The answer to your question is that any subsequent requests or reminders do not nullify the first. The 28 days you have to reply is based on the date of the original.
This is important for two reasons. Firstly it means the six month rule for prosecution is only valid from one date and cannot be moved. Secondly it means that the police cannot make repeated requests for information regarding the same allegation, and thus overcome the six month rule by continually extending the time they have in which to prosecute.
You will have to extend matters until 4th January 26 to force a time out for the speeding offence and I imagine, if you haven't responded by early next month, the police will begin proceedings against you for failing to do so.