freelander TD4 engine noise

Hi all,could do with help, we have a 06 plate td4 freelander 70,000mls that is giving us grief, it starts and ticks over fine but at about1800-2500 rpm it starts to smoke and then rattles and bangs and black smokes as if easy start is being sprayed in,no fault codes. we changed the breather filter and that made no difference, had the injectors checked and it was still the same (they did come out very easily as though they had been out before). we removed the engine and stripped it and decoked the head and valves (VERY BAD!), ground the valves back in rebuilt the engine set up valve timing and had the turbo reconditioned.refitted engine and was fine for 6-8 weeks, now the symptoms have returned.Again no fault codes, even took it to local LR specialist for plug in with their expensive equiptment but again it shows no fault. they did suggest we change the injectors but i am a bit reluctant to do this yet as have seen on forums where people have had the same fault and changed injectors but not cured it. have changed diesel in case it's had petrol put iin and tried crank sensor but to no avail. seperated exhaust in case it was the cat but still the same,one od thing though with it idling if you disconnect the plug on no4 injector it comes up with no3 injector fault and visa versa disconnect no1 code PO200 comes up and disconnect no2 and 0206 comes up. we have only known the vehicle for about 8 months and are unable to contact previous keeper to see if they have any history.Has anyone got any ideas?