Sluggish starting?

Hi, can anyone advise me on how to rectify my recently purched 1998 's' reg 2.5 diesel transit's starting difficulties? I have changed the fuel filter, air filter, but I still have the same problem when trying to start it. It turns about a dozen time's on the key, the when repeated, it starts to cough into life, but it sound like it is waiting for the full fuel flow to get to the engine....! After a couple of seconds the engine is in full 'life'