I had this on my old 2.0l TDCi, i changed the fuel filter, cam sensor egr etc, I took it into a Ford main dealer as i knew someone that worked there but even with there proper Ford Diagnostics SDD, they couldn't tell me for sure whether it was the injectors or the pump. I didn't mind doing one or the other but not one then it turns out to be the other

I had the injectors out, had them leak back checked, came back ok so i had them cleaned and the fault moved from being ok untill the glo plug light came on to being flat all the time and some black smoke under acceleration!
I got so fed up with it in the end, I sold it as spares or repair! if i could've been sure what the fault was, i would have sorted it!