think thats your problem , you dont stretch its legs enough .... diesels need a good clear out . take it up the road or motorway, and give it a good thrash in 3rd and 4th gear , hold it at 3500 revs for a bit and watch all that smoke come out. my cdt dont smoke at all, as i give it a clear out every week .....
also when was it last serviced ? and pcv changed ? also check egr that will be blocked no doubt.