Just a few points which may help.
As zebedee correctly says, the route to go down is to contact the court where the summons is answerable and ask to make a Statutory Declaration that you did not receive the two letters. Beware, that you will have to make this under oath and can be prosecuted for perjury should your declaration prove to be false.
Once your declaration is made the prosecuting authority will have the option to either continue with the original proceedings (in which case they will have to show that the Stat Dec you made was false), or to drop the S172 charge and revert to the speeding matter (which they will only do if you agree to plead guilty to speeding) or to begin the whole process again. They will almost certainly choose the latter and they have six months to bring the prosecution before the court if you fail to respond again to the request for information.
Letters relating to these matters are not sent Recorded Delivery. The prosecution only has to show that documents have been posted, not that they were received.
Far from wanting a �pound of flesh�, Section 172 proceedings are the only way prosecutors have of pursuing registered keepers who fail to respond to the request for the driver�s information. Prosecutions for both failure to give information and speeding are not pursued. Apart from anything else, until the identity of the driver is known no proceedings for speeding can begin. In motoring matters only one lot of points (or a disqualification) for the most serious offence is awarded for multiple offences arising from the same incident.
The only time prosecution for both offences may be considered is if the driver and the registered keeper are different. This is rare because usually by the time the identity of the driver becomes known, the statutory time limit of six months for the speeding prosecution has lapsed.