The diesel fuel injection pump has an integral lift pump which draws fuel from the tank and through the filter, so there's no possibility that the leak is on the fuel supply side as the pump would draw air and not fuel. More likely it's leaking from the injection pump body itself, or the fuel return pipe to the tank as you say it only leaks when the engine's running. If it's an injector pipe broken the engine would be misfiring. Repairing an injection pump is definitely not a do-it-yourself job and will have to refer it to a specialist.....

In that case it sounds like a shaft seal leaking in the pump body. Definitely a specialist job.....