their second photo shows the roundels being on 40 long after we passed them - not before, or just as we went under. Why not?
The first photo of the car is timed 19:54:26 and the photo of the gantry is timed 19:54:27, so I'm not sure "
long after" is accurate. These cameras have a minimum 10 second delay in enforcement after a speed limit change, and if challenged the police will turn up with expert evidence to show how the equipment works. The record also shows the 40 mph limit had been in force for over an hour at the time when you went past the camera.
Are you basically saying that the sign said 50 mph at 19:54:25 or 19:54:26 when you went past it, and it changed in the split second between 19:54:26 and 19:54:27, and the system that is meant to pause enforcement for 10 seconds was faulty, and the record showing when the limit last changes was also faulty?
You can advance that as a defence if you wish, but you will probably want to instruct an expert who can provide expert evidence about how the system could have malfunctioned, because the Crown Prosecution Service will undoubtedly instruct an expert of their own if challenged.
This can be an expensive approach though, see this case: