micra 04 1.5 dci smoking black fumes not egr, could it be cambelt?

I have an 04 reg micra 1.5 dci. it started to smoke black smoke from the exhaust recently and gradually got worse nissan told me it could be egr valve so i had that changed and cost over 300 BUT it has only made problem worse intenswe black smoke so much so that it is now undriveable as it is a risk to other drivers! now nissan tell me it can be the cambelt is this so??? i desperately need some advice as it is costing me a bomb!