There was a popular misconception many years ago, that there was a requirement for speed cameras to be conspicuous in order for the speeding offence to be prosecuted. This was largely due to Tony Blair saying that that was going to be the case. Surprisingly enough, it was a big fat lie.
What the rules actually were, before they were rescinded many years ago, at first glance at least, that in order for the safety camera partnerships to be able to retain the income from the fixed penalties (and return any surplus they hadn't managed to fiddle to the exchequer), 85% of enforcement sites had to meet the conspicuity requirements. This had no bearing whatsoever on the admissibility of the evidence, it was just a rule regarding keeping the money. Except that the rules were enforced by the safety camera partnerships themselves - if they became aware that they were not complying with the rules, they had the power to report themselves to themselves and then decide whether or not they still ought to keep the money.
