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OK for the engine to take an age to start then I'd say its either a lack of fuel pressure or a problem with synchronisation
a few models have a lift pump in the tank and others use a mechanical lift pump on the back of the main pump which is very susceptible to the filter being blocked or a leak before it
So its worth considering fitting a new fuel filter as if only slightly blocked this can lower fuel supply pressure and choke the main pump, the pump will be more sensitive to this when its cold as diesel thickens considerably with low temperatures.
It may have developed an air leak on the fuel supply pipe from the tank to the engine. The engine has no separate lift pump and relies on the built in one in the injection pump. If a leak has developed on the fuel suction pipe then air entering the system will air lock the pump.Try pumping up the fuel using the hand pump in the engine bay on the filter housing, then crack off / loosen each injector pipe in turn while cranking the engine and getting an assistant to operate the primer. This will expel air from the injection side and the engine should fire up. The problem will be cured by checking the supply pipe along its length for signs of leakage and repairing any you find.
If you daub each pipe joint with heavy grease and the issue goes away then wipe the grease away one at a time to highlight the problem joint
if you think this is OK then I'd consider replacing the cam sensor next as its this that controls the injection timing
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