Whilst the requirement to serve a NIP within the 14 days is mandatory, the required details are "merely directory" - if you know which incident the NIP refers to (or if the correct time would not have made any difference), then the NIP is not invalidated for the purposes of s. 1 Road Traffic Offenders Act 1988.
Logically, it seems that the NIP and the paper s. 172 are likely to be correct and the online time is wrong.
*If* the paper NIP has the wrong time, then arguably there might be a get out of jail free on the basis that presumably nobody was driving that vehicle at that location at the time specified, and you are not required to second guess what the police actually should have asked.
If the time on the paper NIP is correct, then as the online form is a mere convenience, you need to decide how bothered you are by the incorrect time. Do you want to eat today, or buy a stamp and use the paper form.
Arguably, using the online form (if that has the wrong time) would seem to provide the prosecution with evidence that you were driving at time Y when they have evidence of the offence being committed at time X, so that should not be sufficient to convict you of the speeding offence - although raising that as a defence would seem to leave you open to a charge under s.5 of the Perjury Act 1911 (assuming that you weren't driving that vehicle at that location at both times).
