I’m told that an officer must ‘form an opinion’ that the car is speeding before taking a camera reading. Is this a law or just an acpo guideline and if so, which law is it ?
It is the law rather than an ACPO guideline if its a manned speed trap. Its r.89 Road traffic Regulations Act 1984. The opinion that you are speeding must be corroborated by either a speed detection device or another officer.