The rules are straightforward.
If he was not stopped at the time of the offence, the prosecutors must send a Notice of Intended Prosecution (a �NIP�) to the registered keeper of the vehicle and post it so that in normal circumstances it will arrive within 14 days of the alleged offence (with the day of the offence being counted as day one). It seems this has been done.
After that the prosecutor has six months to �lay an information� before the court (that is, to start proceedings). It seems also that this has been done (just).
You do not mention whether a fixed penalty offer was sent along with the NIP. If it was not it could be for one of two reasons: either the driver already had nine or more points on his licence and would thus become liable to disqualification under the �totting up� rules, or the speed was excessive and beyond the limit where a fixed penalty is considered suitable. (In a 50mph limit a summons is usually issued if the speed exceeds 75mph).