Raskolnikov

I am in Essex, my car is a 2003 Citroen C5 with the 2.0 HDi and I am getting the dreaded Anti-Pollution fault warning with the accompanying painfully slow acceleration, particular at the lower end of the throttle range. Had it at an approved Citroen dealer last week, had the airflow sensor replaced with a new one. This didn't fix the issue, but left me out of pocket to £350 :( Brought it back to the mechanic, I complained to the manager, who then agreed to have another look at it for free. He found some hosing that was split that he replaced, replaced some filters and did a few other things. Driving it home, the warning light went off again with the same error and the same painfully slow acceleration. At this stage, I am just about ready to give up on this car, but am willing to give it one last chance. Can someone give me a list of what the common things I should be checking out for? The Citroen dealers seem clueless about this issue.