i would have said that one of your fuel pumps was at fault (theres one in the tank and an other in the engine bay) your dealer replaced the high pressure engine pump (Common problem on the 3.0 diesel), but im suprised they reckon its the glow plugs as it should still start in our climate with faulty units.
Once they have replaced them and it hopefully solves the problem, i would question the need for the new pump and see how you get on