I'm sure steve will be able confirm this, but I'm think the downside building was contemporary with the main station building.
there may have been an earlier station further north at the very inception of the line, but that is conjecture.
there was of course the pumping station for the atmospheric experiment which stood on this side of the tracks, so you may be thinking of buildings associated with this.
also, remember that the platforms are above the station, and the steps were, I believe, inside the building....
this[/i], from Falkor, should help;
http://forum.sydenham.org.uk/viewtopic.p...88c7489786