That would be my second question and a curiosity - taking the word "question" in the wider sense, as opposed to pose to the OP.
If the putative charge is only speeding, then if it is fatally defective it cannot [lawfully] be rectified.
If the response to the reminder NIP/s. 172 was not received or otherwise deemed to be inadequate, and there was an s. 172 charge, the time limit to instigate proceedings for that charge would be roughly 4-6 weeks after the speeding would have timed out.
At the risk of generalising, but knowing the propensity of many OPs to seek for advice on very narrow perceived potential loopholes whilst spoon-feeding us slightly less than they think we might need to answer the question, I am wary of diving headlong down any given rabbit hole before I am confident that we have ascertained the relevant facts.
We have also seen numerous OPs swear blind that the only charge was speeding (despite not having named the driver) only to find a second charge on a separate sheet.
