If the police stopped you at the roadside then they will likely have given you a verbal notice of intended prosecution, if this is the case then they have up to 6 months from the date of the offence to issue a summons. It is more likely that they would offer a fixed penalty for this offence, again if this takes longer than 6 months it will be time barred.
If however you were not stopped at the roadside the police have 14 days to issue a notice of intended prosecution to the registered keeper of the vehicle.