This is a common rail diesel engine, I find in the higher gears the engine miss-fires and there appears to be unburnt fuel emitted

from the exhaust. I thought it might be a dirty injector but I have treated it with injector cleaner but it made litte difference.

Does anybody know what the problem may be or the best way to diagnose ? 

I still think that you may have a injector problem perhaps the cleaner couldn't cure ???

Agree with O.T.H and could also be the turbo emitting smoke but also build up of carbon that will eventually clear if revved enough over a period of time

ODBII reading of P0251

My car does not run smoothly, it hesitates periodically in any gear. I assumed it was
due to blocked injectors but I removed them and cleaned them physically and also with
injector cleaner additives. Despite that the injectors became covered in soot. 
I assumed the reason could be a dirty ERG valve so I removed it and found there
were large soot deposits such that cleaning the injectors was pointless when there
were deposits of soot awaiting entry. Despite taking the above action the problem has
not gone away. I have a ODBII reader and plugged it into the socket. It gave an error
reading of P0251.
P=  Power train (engine/transmission)
0   is generic
2=  Fuel and air metering (injector circuit)
51= DTC two digit number as described in 2

There are a number of components that could be the cause of this failure, has any other
memeber experienced similar problems and can advise on the best approach to solve it.