Nissan Qashqai Diesel 2014my lack of power

Nissan Qashqai 2014 my
Vin: SJNFAAJ11Z1084466
Engine is K9K 636 which is a 1.5Dci turbo

Vehicle history:
Car was traded in at a dealership. They noticed a severe lack in performance and assumed the head gasket to be damaged.The dealership sent the car to the local engineering shop to have the head gasket replaced upon removal of the head they noticed no damage.They assembled the car and it does exactly the same and now the car is with me.

there is only 1 fault code which I picked up with the launch.

P0380 Preheating unit DIAG CONN
I dont know what this fault means and I am not sure whether this is the fault for the lack of power.

I have checked for air leaks(smoke test), The turbo is free.

Please can I have some direction

The car drives fine but there is the lack of "go" when you accelerate almost as though the turbo is not working.

Regards