The first point of call on a diesel thats hard to start and spluttery from cold, is the heater plugs / glowplugs.
If the air in the cylinders is colder than it should be the engine will not start, the cranking for longer gradually heats the air charge by pump action and the engine starts but runs roughly until its reached a minimum operating temp in the cylinders. Working glowplugs super-heat the air in the cylinders directly while the orange light is on, to make the engine start quickly and easily.
Try putting the ignition on for a few seconds then off a few times to get maximum heating effect from the existing plugs (when cold) and see if it starts easier, the repeated ignition use builds the heat in the plugs when they arent too good and improves starting in most cases, unless more than one glowplug is completely dead. (broken link cable between them? can happen)
They are cheap usually and fairly easy to replace.
(The other reason can be low fuel pressure to the injectors at cranking speeds, which are typically about 300 to 400 rpm, and when it starts the revs lift to 1200 ish, producing more average fuel pressure and improving combustion, as vacuum is weak when cranking compared to idling once started, and a vacuum pump drives the fuel pressure regulator on a common rail diesel engine, and diesels don't have vacuum in the inlet chamber at all unlike a petrol engine, so the pump runs slower during cranking.)
Check for fuel leaks from injectors and pipework, faulty vacuum pipes to the FPR, faulty FPR, or poor / faulty glowplugs.
