To prosecute the driver for the alleged speeding offence, they would have to instigate court proceedings within 6 months of the date of the alleged offence. Assuming that the date you received the original NIP was after the date of the alleged offence, it is already too late to do so.
Just as a sanity check, this "same NIP" that you have received, does it say "Notice of Intended Prosecution" at the top, and/or contain a warning under s. 1 Road Traffic Offenders Act 1988 that it is intended to instigate proceedings against the [unnamed] driver of that vehicle for exceeding the speed limit in <vehicle> at <time> and <place>, or does it say "Single Justice Procedure Notice" at the top and ask you to enter a plea within 21 days?