Changing the fuel filter on a 300tdi is an easy job on the disco 300 tdi. Takes no more than 10 mins. You literally just unscrew it and screw the new one back on. I used an oil filter wrench to get mine off as it was secured quite tightly. When I put the new one on, I just tightened it with my hand and didn't use the wrench, that way it will be easier to get it off next time.