I've just had this very problem. Mine was also accompanied with a judder on over run around 2000-2250 rpm when I backed off gradually on the accelerator. From a stand still it looked like the end of the world in my rear view mirror because the smoke was getting gradually. I took the usual wishful thinking steps by servicing. Next step I took the egr valve off and then the inlet manifold and cleaned the inch thick layers of carbon off and put it all back together. Took it for a spin. Still the same. Only slightly less black smoke. By chance I checked the intercooler hoses and found a split. Not all the way through but being held together by the weave inside the hose. Replaced with a silicone one and took out for a spin. Power totally restored no smoke no surging no juddering on over run and more importantly. Mpg up from a terrible 44.2 to an impressive IMO 54.6 mpg. All for a 20 quid silicone hose from flee bay.
Thanks for all your help & advice Stevo.
I have a Haynes manual so I'll find out where the intercooler hoses are & check them out. I get 46-48mpg usually so I'd appreciate an increase to 54mpg.