Need Help Please - 300D fuel injector leak
 

I need help diagnosing my fuel injector problem and looking for any repair advice. This is a new issue for me so if there is anything I'm missing/not asking about I would greatly appreciate any advice.

First I have an '82 300d (european model imported from germany if that matters). Fuel is leaking at the junction of the metal fuel lines and the square hexagonal nut at the top of the injector where it meets the cylinder head.

The picture below (not of my engine) identifies the location and injector/cylinder that is leaking.
http://dl.dropbox.com/u/36056830/1.png

This picture further identifies the exact location of the leak and how it is happening (fuel is leaking, in time with engine rpms/fuel pumping, from between the metal fuel line and the thin metal collar above the nut that seems to be at the top of the injector)
http://dl.dropbox.com/u/36056830/2.png

This problem started at the tail end of an extended road trip and once the car cooled down it is very difficult to get the car to start and it will only run with the throttle opened as very little fuel is getting in to the affected cylinder.

I have zero experience with this issue. I don't know if I just need to tighten that hexagonal nut to get the fuel line tight like the others, or if there is a ring or seal that needs to be replaced, etc etc.

Any and all help would be greatly appreciated. Also new to this forum and couldnt find this problem already addressed so apologies if this is redundant at all.

ForcedInduction -- thanks for responding. I tried tightening the nut but the line remains loose. I tightened/loosened them on other cylinders to make sure I was doing it right as well. Is it possible the fitting has worn or for some other reason won't seal to the injector? Any idea if that would be the nut or the fuel line (or that little disk inside the cylinder...inside the nut...that the fuel line sits on?) that is at fault and might need to be replaced? I will certainly try to play with it some more but it seems like it just won't tighten up on that fuel line at all.

Will try that, thanks again