Sometimes though, as the authorities who look after these cameras dont have enough time to examine each picture individually, someone might look at the photo, see your numberplate (assuming that the fiesta was close enough to you for you both to be on the photo) and send out letters to both of you.
This happened to me, on a two lane 30mph dual carriage way - i was doing dead on 30 in the left hand lane, when a police car came roaring past, lights flashing, overtaking me at about 60mph just as we were passing the speed camera. Of course, it flashed and I recieved an NIP a few days later. I requested to view the photos, and sure enough there was the police car as well as my car. I wrote back explaining this, and then had another letter apologising, and saying something like "due to the volume of alleged speeding incidents, it is impossible to check all photos".
So, if you do get mail, id write back, request the photos, and send a nicely worded letter explaining what happened.