Firstly a point of clarification. The maximum permissible speed is not the “speed limit + 10% +2mph” as you suggest. The maximum permissible speed is the speed limit, full stop, and you can be prosecuted for exceeding that limit by any amount, however small. The “10% + 2” allowance has been set by the Association of Chief Police Officers to help the police and Safety Camera Partnerships in their decision whether to initiate prosecution/fixed penalty action or not. It is designed to make an allowance to reduce the number of frivolous defences which challenge the calibration of instruments, etc.
Now for Section 172. If you believe you cannot identify the driver at the time of the alleged offence you must decline a fixed penalty for speeding (if offered) and wait for a S172 prosecution to be initiated. You must then go to court and should plead not guilty. Your defence will be that you cannot reasonably be expected to be able to identify the driver (because of the time delay you mention). It will be a matter for the court to decide whether you have exercised “due diligence” in your attempts to identify the driver.
By the way, I don’t understand why the NIP took a long time to reach you as they are sent in the first instance to the Registered Keeper, which you say is you. If you are using the time delay as part of your defence, this may be something to your advantage but it depends on the circumstances.