http://www.speed-trap.co.uk/Accused_...ge/The_Law.htm
scroll down to NIP and read.
You need to understand the law and your rights and be prepared to fight and stand up to the fookers....
I would post my own experience as regards to the 14 days rule but my posts are mostly ignored so can't be arse.
Some pointers:
This last section should be of particular interest. It means that if the police spent a month just trying to find a record of the car's owner, that month doesn't count as part of the 14 days. Similarly, if your dog tore up your mail with the NIP in it, that too is no cause to bar the conviction.
Take note of the date on the NIP and the date of the conviction.
If it genuiely arrives after 14 days then you will need balls to fights it....!