It very definitely is a real situation; have seen it with my own eyes. Mind you, been flying since 1986! Appreciate that there could be problems in assuming that it was only the transmissometer fogging-in but that was our situation indeed, and we could see BRS 27 from many miles away and could also see a small patch of fog obscuring the area south of the runway. This the transmissometers were giving very poor RVR measurements whilst the rest of north somerset was basking in a glorious evening sunshine. The controller, however, was a 'numbers man' and had not recently looked out of the window (am being kind here!). It took some persuasion before he finally relented and stated that the machine must be broken! This saved me a difficult decision as I was new to command and I had a right seat recently demoted captain who was urging me to land anyway! The ATCOs late decision saved me from a tricky call.....