are they s/hand or new parts, have they been coded to the ecu ? Does the intank pump work ? Is the pressure sensor on the rail working and is the loom in tact to the sensor ? and the pressure switch on the pump ? most common fault.

Smoking and low power is normally the EGR valve, but if the injectors have been fitted but not coded to ECU it will not start !

We need more information really! When does it smoke; under high load, or low load at idle? What colour is the smoke. Are the new injectors/fuel pump KNOWN to be in good condition, or are they sourced from the scrappie. Nothing wrong with most scrappie parts, but it can play havoc with 'diagnosis by substitution' game. The fact that it works well at higher speeds indicates that there may not be anything fundamentally wrong with the engine. Have you tried the obvious (and free!) things like the EGR valve...?

Thanks for reply. This work was done by an independent diesel specialist. No faults showing on computer. Pump and injectors were new. and programed.

I t. smokes on accelleration and even with a slight pull at 50 mph. Smoke is black/grey.