Regarding the 14 day rule, the legality and time scale in which a fine must be issued is generally similar to being a registered keeper although there are differences that mean the police are given a lot more time in order to gather the correct information they require to make a prosecution. The NIP (notice of intended prosecution) will be sent to the registered owner who will be expected to declare who the driver of the car was within 28 days. If this is not adhered to then the owner could be subject to a hefty fine and penalty points.
The same rules apply for drivers and owners regarding the issuing of a police notice who have 14 days to send this to the registered owner. If the date on the letter is outside 14 days from when the alleged offence happened then you can contest the fine as the driver or owner, and have the fine quashed. If the date on the notice that arrives is outside this time frame you have a case. If however, the letter arrives after 14 days but is still dated with that time, your legal argument will be null and void.
See the Registered keepers Page for full details on the 14 day rule.