Thank you for your response Jo,
The issue is that I was clocked by a MOBILE camera van, looking towards me. this van was parked in a lay-by, only 220 yards from a change from 50 to 30 mph.
The speed limit sign for 30 mph is on the apex of a bend and was COVERED BY FOLIAGE and I only saw it when less than 50 yards away.
The sign on the other side of the road was obscured by traffic. My understanding of the law came from my father avoiding prosecution in the 70s because the speed trap was too close to the change in limit.
At least for him the signage was not obscured and it was a straight road!
I am afraid that your last response has devalued your previous answer.
In the 1970s - as I found to my cost, there were portable speed traps, often disguised as GREEN GPO (telecom) cable junction boxes. These were triggered by the passage of a vehicle over pneumatic cables that were laid across the carriageway.
Speeding excess, captured by mobile traffic offices in a car, was over a measured distance and verified using a specially calibrated ADDITIONAL speedometer in their vehicle.
this continued at least into the 1980s when I worked for the Talbot Motor company, with specific responsibility for the sale of cars to a number of Police Forces.
So, I am now dissatisfied with your earlier answer. You might (as I do not know your age) possible have travelled in a car in the 1970s or 1980s, but your answer shows that you do NOT have adequate knowledge of that period. Therefore I am unconvinced as to your actual knowledge to answer the question in hand.
If the stipulation that I am aware of DID exist in the earlier Road Traffic Acts, it can only cease to exist now, if those acts have been repealed.