When a car has been flashed by a speed camera or caught by another speed device, the registered keeper of the vehicle should receive a Notice of Intended Prosecution (NIP) within 14 days. The courts will allow a couple of extra days for postage. If nothing has been heard by the registered keeper then it is unlikely you will receive a speeding penalty notice or summons.
If you do receive a Notice of Intended Prosecution then Section 172 of the Road Traffic Act requires you to return the NIP Information form with details of who was driving within 28 days. If you do not know who was driving you must state this. However you can be liable to a 6 point fine if you do not return the form or provide the information. Completing the form with incorrect information is a serious offence.
If the registered keeper was not the driver then the police are allowed more time to establish who is the driver. This can be where it is a company or hire car or works van or if the vehicle has recently been sold and the new keeper’s details not yet registered with DVLA.
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, in which case a written NIP is not required. If there is a significant mistake on the Notice of Intended Prosecution, then you may have a valid defence. However, under the slip rules, the court can make minor amendments if it is only a small or incidental mistake.
If you only recently acquired the vehicle, then the police may send the NIP 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.
The police have 6 months to issue proceedings, which may be by way of fixed penalty notice or for more serious offences summons.
The fine for speeding is £100 when dealt with by way of a fixed penalty notice.