Black smoke from diesel

[bodgit]

I know its normal to get black smoke from a diesel but yesterday my 2.0tdci cmax 55 plate was blowing a hell of a lot out, cars behind were dropping well back and my tailgate is covered in crap. I did a round trip to Cardiff 300 miles and noticed under load it spewed it out in large quantities. Car has never had any engine faults before and has been serviced according to book specs. Recent oil/ air filter done 1200 miles ago. I did notice little bits of smoke before but thought it might have been because the dealer overfilled the oil.
Anyone got any ideas what may be the problem. Up to now Im thinking faulty injector or turbo seals but have no real idea.