The Merseyside camera partnership claim they have evidence that i exceeded the temporary speed limit of 50 mph at junction 26 (haydock).
The only camera there is a specs camera and there is only one after this the road returns to national speed limit, as specs is an average speed camera system can they successfully prosecute and convict based on just the one camera as you cannot take an average speed with just one camera as average speed is calculated by speed vs distance.
Sometimes they have an officer with an LTI 20/20 sat at SPECS camera sites.
You are right that if they had only one camera they would not be able to ascertain an average speed for the distance covered between two points.
Why don’t you call them up and ask how they claim to have detected the speed for your vehicle. I suspect it was a different kind of device.