Do the forward facing fixed place camera’s still flash when they have caught you or is it a ‘silent’ approach. 
Yes they do flash but this is only a "dummy flash’.
The speed is measured using pressure pads mounted under the road surface
The cameras you refer to are made by Truvelo. An explanation of how they work can be found on their website at www.truvelo.com
It’s not a “dummy flash” - It’s used to take the photo, allowing a sharp picture to be taken that will “freeze” the motion of the vehicle. There is however only one flash (unlike the rear-facing cameras) as the multiple speed sensors corroborate one another instead of relying on two photos taken over a set time interval.
Rear facing cameras use a single radar device to measure the speed, and this reading is corroborated by the two photos and the lines etc painted on the road surface. Front facing cameras use multiple speed sensors in the road surface, so the photo is only required in order to identify the vehicle (and of course the driver, being as it is front-facing).
With both types of camera, the box may or may not have an active camera inside it (which may or may not have reun out of film). Both types can be fitted with a dummy flash unit instead of a proper camera.