Did you know that every time your car goes past a speed camera, even 1mph over the set limit, it is registered and put on a database? You wonly get a ticket if you are way over the limit or, (this is the bid you did'nt know) if you receive over 10 near misses, you will be classed as a serial offender and get a ticket the next tiem you go over the limit
This is why you hear of people being done for 34mph in a 30mph limit, whilst others doing 39 do not.
You can check what has been registered against your vehicle at the following web address:
http://www.i-database.co.uk/index1.php