You need to have at least one fog light positioned centrally or to the offside of the vehicle. It cannot be positioned within 100mm of the stop light. If the fog light is part of a common housing then the separation criteria is assumed to have been met if the housing is ‘E’ marked
It must be red in colour.
It must be a minimum of 250mm from the ground and no more than 1000mm. (measured to edge of light unit)
It must be E marked to B or F or show equivalent standard
Some failure points to note;
a. has no "tell-tale" light fitted
b. has an inoperative "tell-tale" light, so that it cannot be identified
c. has a "tell-tale" light that cannot be seen from the driving position
d. operated by a brake control.
e. fitted such that the reflector is not facing squarely to the rear.