2.5 dci lack of power

Hi all new to the Renault scene and got this traffic to build into a day van show the dubs how to go on....



Onto the subject in hand.

Engine has intermittent starting lacks power and cuts out under load sent it to a diesel specialists and they said fuel rail was dumping the pressure so fitted new one is abit better but still same fault spoke to them other day said they think could be fuel pump but also said no fuel is being returned could all these be down to pump creating not enough?? I am at end of my tether with this and help be much Apreciated.



Thanks