Nissan Primastar with no power?

Please help. I have a nissan primastar 03 plate dci80 which has lost its power. It started by just having no go when cold but now it really struggles. When you set off from cold there's no power, only enough to get you moving and wont rev high enough to reach the turbo, untill warm. The turbo usually comes in about 2500rpm and most of the time makes little difference at all and if you dont get the revs high enough i'd have to change down a gear to get up a hill. (2nd & 3rd) please help.