Did some work with Speed camera partnership in my area for uni. ACPO (association of Chief Police Officers) dictate that you only get prosecuted if you are over 10% + 2. I.e. in a 30, it is 35. This is why you never see a letter of prosecution for doing 31 in a 30.One in six cameras in Kent have film (even though they flash), not sure bout other areas tho. I got flashed by red light in Croydon but never got a letter. Bottom line is you either have to be speedind like a c*nt and deserve it or extremely unlucky to get points.