If you receive a Notice of Intended Prosecution through the post, then according to Section 172 of the Road Traffic Act you must return the NIP Information form with details of who was driving the vehicle at the time of the alleged speeding offence within 28 days. See Defences, if you do not know who was driving. However be aware that you can be liable to a 6 point fine if you do not return the form or provide the information required. You should not ignore the NIP even if you think that you have a valid speeding defence.
If you are not the registered keeper of the vehicle then the police are allowed more time to establish who driver was. This may occur for instance if you are driving a company or hire car or a works van or if the vehicle has recently been sold and the new keeper not yet registered with DVLC.
If you are stopped by a police officer, or there is a police officer at the scene of an accident, the police officer may give you a verbal NIP, and if this is the case a written Notice of Intended Prosecution is not required. So pay careful attention to what a police officer actually says in such a situation.
If there is a significant mistake on the Notice of Intended Prosecution for speeding, then you may have a valid defence. However, under the slip rules, the court can make minor amendments to the NIP and you will have no defence in these circumstances.
If you have only recently acquired the vehicle, the police may send the Notice of intended prosecution to the previous registered keeper, who will then of course inform them that the vehicle has been sold. If this error is your fault (eg for not sending in the registration document with your details) then again this will be no defence. See Speeding Defences.
The police have 6 months to issue proceedings, which may be by way of a fixed penalty notice or summons depending on the circumstances of the alleged speeding incident.