found this on a legal web site Defences that are not valid None of the following can be used as a defence against a speeding charge, even if you felt that you were justified in speeding at the time. the camera did not conform to the regulations set out for their operation there were no relevant visible road signs the speed trap was hidden – there is no requirement for the police to make themselves visible or provide warning when operating in this way you were late there was an emergency the car behind was too close. which then seems to be contradicted by this on the same site. Rules and regulations for speed cameras In 2001 the law was changed so that speed cameras had to be painted yellow to ensure visibility. Many other rules were also brought in regarding speed cameras. The regulations decree that: speed camera housings must be coloured yellow camera housings cannot be obscured, e.g. by trees, bushes or signs cameras must be visible from 60m away in 40mph or less zones and 100m for all other speed limit zones signs must only be placed in areas where camera housings are located or where mobile cameras are in operation in order to make them visible, mobile speed camera operators must wearing fluorescent clothing, and their vehicles should be marked with reflective strips camera sites are to be reviewed at least every six months in order to ensure that cameras are adequately visible and signed. The presence of speed cameras can also be indicated with road signs, but this is not a legal requirement. but i think the second set of rules is mainly for fixed sites.