If a driver has exceeded the speed limit, the registered keeper of the vehicle should receive a Notice of Intended Prosecution (NIP) within 14 days. The courts will usually allow a couple of extra days for postage. If nothing has been heard by the registered keeper after 16 days or so then it is unlikely you will receive a speeding ticket.
If you do receive a Notice of Intended Prosecution then Section 172 of the Road Traffic Act requires you to complete and 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 was the driver at the time of the alleged offence. This may be where it is a company car 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 have 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. For offences at higher speeds you will need to go to court and if found guilty will be sentences according to the sentencing guidelines.