If you go through and get flashed once this is the camera performing self test. happens even if you go through below the limit. Two flashes are a recorded image.
As mentioned takes two images and will be quite clear who is speeding.
Gatso cameras work on film however these are now being replaced with "Monitron" which are directly linked to the ANPR database and will issue your letter imediatly and will be in the post and on your mat within 14days. The road markings are the same as Gatso's however the Monitrons have smaller Cameras and mounted on higher posts. Monitrons work off induction loops embedded in the road, Gatso rely on radar.
Monitrons are also being posistioned at T.L's and will clock you through a red AND speeding through the junction. Have seen a camera mounted near by flash many people speeding through the junction on a green light.
The + X mph factor is completly down to the posistion/location of the camera and police force.
You should be ok if someone was overtaking you...
