White smoke on a diesel engined car is more likely to be steam/water comming from the exhaust pipe. Black smoke is unburnt diesel.
White smoke could be down to the fact that there is a build up of water in the exhaust system (do you do lots of short runs) Good long journey of 20+ miles should resolve.
But first check water level in the expansion tank for the cooling system - if this level has dropped ie loss of water then there may be a head gasket problem. Take car to a garage and get them todo a check for exhaust gasses in the expansion tank.
Maurice
December 2009