Far as I know, the police are now required to send a Noitce of Intended Prosecution (NIPS) to speeders within 14 days of the offence. Any longer and they cannot prosecute.
Presumably this NIPS was also sent to the wrong address? If It hasn't been received and the registration document is correct, then it is their error and after 14 days they are unable to do anything.
Ignore it until they come round for you. Then ask to see the original paperwork and point out their mistake. Point out that they have not served you with the required NIPS and the matter is therefore time expired, and rtell them that they will have to write to you with an explanation of their error and setting out their intended action