my xtrail started loosing power but only when id pull away after being stoped with the engine running like at traffic lights or a junction when id pull off it would not go above 2000 revs until id turn off the engine and start it up again then it would drive fine till l had to pull up again nissan dealer told me contaminated diesel had ruined fuel pump and injectors l have my doubts

Why did they come to that conclusion? Are your pouring cooking oil in there or running it on recycled chip fat.
If not then they are just trying to get a quick grand out of you.

l would try some snake oil additives and a witch doctor. Actually a new diesel fuel filter some fuel moisture cleaners and injector cleaners would not hurt anything and cost much less. Also check and replace the air cleaner element if necessary. Fresh engine oil? And really good name brand fuel from another fuel station.

Not entirely true l am afraid, Nissan know that there is a problem with the high pressure pumps on T30 X-trails, the pumps can & will wear prematurely however filling from an old diesel can that could be dirty will not help. The pump creates 230bar at idle & does not like even the smallest contaminates, dirt as small as 10 microns is enough to ruin a pump. l would ask to see a print out from there Consult 2 or 3 machine that confirms there findings, if it is what l think it is the fault code probably will be P0089 & this is related to the suction control valve, which can be replaced with the pump still in the car.
You car still runs, sometimes on default which is 2000rpm & will drive at a lesser speed which tells me that the pump is still creating enough pressure to maintain injector pressure which is then controlled by electronics. When the suction control valve fails, the suction control valve cannot maintain the flow to the pump & it starves the pump of fuel which is why it goes into default ie limp home.
If the engine starts but at idle the engine hunts ie the revs rise & fall, then it is a suction control valve, Nissan dealers will have the part number available, In my experience, l would advise replacement of pump & the tech would then reprogram the pump learn values.
Hope this helps.