What is the green LED under the rear view mirror telling me and how do I kill it?

That your auto dimming mirrors are on.

Press the button on the base and it goes off

I think that I switched of the mirror on my wife's 2003 MY Polo 9N very soon after she bought it, then was going to report the failure of the auto-dipping mirror at its first service, the weekend before the service I pressed the switch and heh presto green LED on and the auto-dipping function works again! Well that avoided an embarassing exchange with the dealer!

What is the green LED under the rear view mirror telling me and how do I kill it?

Its telling you to read your manual

Mine dosen't have a green LED or a button to press, the manual just says it automatically dims....and it dosen't have a switch at the bottom so it is definatly auto dimming.

It will have a sensor on the bottom, if it hasnt then it isnt auto dimming.

Yup, unfortunately the early ie B5 (and maybe even the later B5.5) Passats did not get a switch on the auto dipping rear mirror, so it looks like the Superb inherited the "early variant" of this option. I can't make up my mind if this is good or bad - under low light level conditions, especially where there are street lights, it is quite easy not to be aware of all following cars that are only using sidelights, while the auto dipping is operating (having been activated by a following car with headlights on. I know that driving on sidelights is not legal nowadays and that UK was forced to dump DIMDIP, but it still makes driving at night, in town with non-switching auto dipping rear mirror a bit more risky that was intended - and anyway why should my wife's car (Polo 9N) have yet another switch/function that my (Passat B5) hasn't!!!