Can you put a few pics up from different angles of the motor?
If the motor runs and drives with the same amount of power and is as smooth as a TD42 motor can be, then I'm thinking that it will be a return line from the injectors back to the tank. I say this assuming it is quite a serious leak which if on the inlet side of things, will reduce the diesel pressure going into the injector pump and or injectors. I doubt very much that it is a leak from the injector pump to the injectors as that is under a much higher pressure, but stranger things have happened.
Like Alex said, get under the motor and follow the fuel lines from the tank to the filter and then to the IP. Also check from the spill rail ( the small metal tube thingy that connects all 6 injectors together ) as this sends the unused diesel back to the fuel tank. There is also a small rubber hose if I recall that connects the spill rail to the return pipe to the IP ( which will via the IP send the unused diesel back to the tank ). Age and heat could have degraded that piece of pipe causing a leak. This small pipe is a little tricky to see, as it fits in between the intake manifold ( between no 3 and no 4 cylinders if I recall ) and connects to the top near the back of the injector pump
This is the pipe I'm talking about....
And this is where it fits onto the IP.....
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