it could be an injector sticking open, a valve not seating or excess oil getting into the intake and with blue/grey smoke Id be more inclined to lean towards oil over diesel being burnt in excess.
pull the hose off of the front of the intake manifold from the intercooler, see whats in that and start it up without the hose to see what it does.
if you've got or have access to a suitable nanocom or similar that will give more clues.
TD5 injectors are pretty reliable units on the whole most failures being the top seal letting diesel past into the cam area of the head diluting the engine oil.
TD5s also have a fantastic filtration set up so its not unusual for the oil to be cleaner than you expect for a diesel.
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