As I was watching the Discovery show (on the work on getting the Concorde back in the air), I noticed that there, besides the Captain, F.O. and F.E. also was a 4th crew member situated in the corner behind the Captain and the F.E. (draw a line from their back, and you know what I mean).
What is his task? I never saw him involved in any procedures.
He can also be seen on this image
Click for large version
Photo © Richard Paul