the answer is simple... standard Td5's are all sluggish at low revs... , get some live data to make sure all the sensors are within limits first, put new air and fuel filters, get rid of the EGR if still there, if it's EU3 fit a de-cat downpipe, get it remapped...then you'll regain your smile
clean MAF, MAP(inlet manifold) and AAP(top of airbox) sensors untill then. the leaking filter doesnt affect acceleration.