[RoverP480]

A number of years ago I went to Malvern Diesels, about my poor running Land Rover, and the first thing the mechanic did was change the fuel filter. I protested that I had only changed it a couple of weeks previous. He stated , that he always changed the filter before doing any other analysis , as it took that out of the equation. He then opened mine up with a can opener he had on the wall, it was totally black and blocked. Lesson learned and problem solved.
I worked for BMC/British Leyland/ Rover/Land Rover and we had a lot of trouble in the late 70's with warranty claims on diesels due to blocked filters . The problem often turned out to be due to fitting none original cheap replacements , which although the right filter pore size , had less than half the area so blocked up quicker. One way of making them cheaper!
Since then I change mine each year irrespective of mileage and sooner if I feel necessary