GU 3lt problem

Hi all was hoping someone could shed some light on a problem that we are having with our 3lt Patrol. We did a trip in June to the NT and through the Kimberlys, on the second day of our trip the engine light came on and according to the manual it was either water in the diesel or a minor engine problem. We had it looked at in Alice Springs they said that the fuel filter was blocked up so we had it replaced, we figured that was the end to our problems. The rest of the trip the vehicle did not seem to have full power i thought i was imagining it but since we have got home we have noticed it slowly getting worse and then on Wednesday my wife rang me to tell me that the engine light had come on again. We have also noticed that when you take off from a standing start even if you plant your foot to the floor there is no power untill you hit about 40kmh which is very unnerving if you are pulling out of somewhere into traffic. We took it to Nissan on Friday they had it all day and not only did they not fix the problem but they have made it worse, they told me they replaced a sensor and the valve that is in the intake line from the air filter but they could not fix the problem and do not know what it is so it has to go back in on Monday so they can try again, in the meantime we are driving a car that is very underpowered and is chewing through the diesel we are only geting 650km out of a tank compared to the 950km we normally get. I hope someone can help as we are getting very frustrated.