If the radiator light is now on, top the water up A.S.A.P, However when water finds its way into the combustion chambers of the engine it can produces white smoke, I hope I am wrong but it sounds like the head gasket is on its way out to me(however without seeing the car, I have made that judgment off your description so I may well be wrong). Finally does it have clean diesel in it? I know for sure that when red diesel is chucked into the tank it can produce excess smoke. Run the tank low, sling some injector cleaner in her and fill the tank up then go for a blast. The above will have one of two effect, either solve the smoke, or if the head gasket is on its last legs, this will put the final nail in the coffin, and it will smoke a lot worse. Finally if you intend to keep the car, remove the swirl flaps before the engine swallows them! Probably not the sort of news you wanted to hear but personally I would take it back!