It seems from what you say that you had your Notice of Intended Prosecution (NIP) at the time of the offence as you were actually spoken to by the police. A written NIP is only necessary when you are not stopped at the time (e.g. caught on camera). In these cases the prosecuting authorities have to post a NIP to the address of the registered keeper of the vehicle so that in normal circumstances it will arrive within 14 days.
Since you have already received your NIP the prosecution must now begin within six months of the offence. �Begin� means that you must be summonsed to a court hearing that takes place within the six months. Presumably that is what you now have.
You must respond to the summons either by appearing in person at the appointed time and place or by entering your plea in writing. If you fail to respond to the summons (either because you did not receive it or any other reason) the case will be heard in your absence. After it has been heard you will be notified of the penalty imposed by the court.
If you wish to dispute this penalty on the grounds that you were not able to defend the case (either in person or in writing) it will be for you to persuade the court to re-open the case and let you enter a plea. They will not do so unless they are convinced by you that you did not receive the summons or that, if you did, you had a genuine reason for failing to respond.
If I am right in my assumption that what you now have is a summons and not a NIP, then you are in a different situation to that described by compostella. Simply ignoring the summons will result in the action I described above.