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POP
There is a time frame for all offences.
If you were stopped at the side of the road and reported for summons (as opposed to being given a FPN) then you would have been given a verbal NIP and a report called an HO/RT10 will have been submitted with the evidence.
Information must be laid before the courts within 6 months (this means the court needs to be told that you are being considered for prosecution), if not the the case becomes time barred.
If you think that you may have been seen by a copper or a camera, then what is called a Notice of Intended Prosecution (NIP) must be sent t the registered keeper within 14 days of the alleged offence.
If no NIP is sent, then again a prosecution cannot proceed, or more to the point you have a statutory defence against prosecution.
There are various other scenario's but they are the 2 main ones for speeding offences, but by and large, after 4 months if you have not heard anything (providing you have not moved address or changed vehicles or anything like that) you can probably consider yourself safe of any further action.