why do i get smoke ,Renault Laguna 2.0dci estate?

for the last 4 weeks ive noticed at night that every time i accelerate i get a lot of white and black smoke coming out of my exhaust. i put some forty diesel treatment in the tank but that has not helped also my fuel consumption has come down by about 6 mpg. My car is a renault laguna 2.0 dci 55 reg estate initiale

affy
October 2010