freelander TD4 engine knock

[tdeedoff]

hi all, we have a 06 plate freelander TD4 70,000 mls which is giving us grief. At 1800-2500 rpm it starts to miss and smoke and then rattles and black smokes as if easy start has been sprayed in, if you are driving it and holding the revs constant it will rattle and black smoke and seem to accelerate, no fault codes present. Tried the breather filter first - no different, had the injectors checked -still the same (they did come out very easily though had they been out before)The engine has been stripped and decoked (valves and head really bad), turbo recoditioned, valve timing reset.put back in and ran fine for 6-8 weeks then symptoms returned. plugged in and again no fault codes,even took it to local LR specialist who said had never heard anything like it before but suggested we change injectors - a bit reluctant to do this yet as have read on other forums where people have had same fault and changed injectors and hasn't fixed it so gave up. we have changed diesel in case it has had petrol put in it and tried crank sensor, one odd thing is that when idling if you disconnect no 4 injector plug it shows no 3 fault and visa versa if you disconnect no1 code P0200 comes up and if no 2 disconnected 0206 comes up. we have only known the vehicle for 8 months and are unable to contact previous owner for any info.Has anyone come across this before?