Engine isn't running smooth

Hello Everyone.
Can someone help me.
I bought a LandRover Discovery Diesel from 2005 and a friends think the engine is running on 5 cylinders. Its a Diesel TD6V 2.7


I went to the garage (not Land rover) and they say it can be one of the valves (Cylinder)
Others say that it can be the EGR valve and advise me to use the Blanking kit.

Does someone have this same trouble and what can be the solution.

Thanks guys
Bas