Hi after proving that we couldn’t find anything wrong with the injection system I asked a local diesel expert to look at the vehicle, they couldn’t fault it. They said it’s only a suggestion but it may be a DMF, so I had a long road test and realised that although the engine was rough, there was no lack of power.

We have changed the clutch & DMF and found the centre bush had hammered itself out of position and was just starting to foul the clutch. The symptoms were so like a misfire, but I suppose the fact that we were not getting any fault codes should have been a giveaway. Occasionally when driving; dropping the clutch and being rough with it smoothed out the engine. Many thanks for your help.
John Kennard.