Good news then. They are time barred.
The rules are contained in S1c] Road Traffic Offenders Act 1988 which you’ll find here
They must either deliver a NIP verbally at scene or serve it upon the registered keeper within fourteen days of the offence.
Under S1[1A] the notice should actually arrive within 14 days.
However, its important to respond in a particular way. The document you have received is confusing. Although it is one document, it contains both a NIP and a S172 form which are two different legal entities. They may well be time barred on the NIP but that doesn’t mean you can avoid your S172 responsibilities.
If you don’t respond naming the driver on the S172 form they will charge you with failing to identify. Probably they are hoping you will do exactly that so that they can prosecute you for something.
If you do fill out the forms naming the driver then its likely you would hear no more about it. If they do seek to prosecute you then you can rely on the fact that the NIP was out of time.
Hope this helps. Please let me know if you need more information.