You need to get in touch with the court that issued the summons. You will need to make a "Statutory Declaration" under oath to say that you had not received the earlier letters. The organisation that issued the original fixed penalty (probably a "Safety Camera Partnership") will usually then set aside the action they have taken to prosecute you under Section 172 of the Road Traffic Act. (This is the section that deals with failure to give driver's details). If they do so they will start again by offering you a fixed penalty notice for the original speeding offence (providing you are eligible).
If they do not (and they may not if they are approaching the six month time limit for prosecuting these offences) you will have to attand court, plead not guilty and stand trial under Section 172. Your defence in those circumstances will be that you did not receive the original notices.