Howdy: I missed what sort of 300tdi you had ( Disco or def). In addition to all the other useful tips the other have given, this was somthing I had a prob with in my tdi disco. It was hard to start in the morning, and was OK the rest of the day. It was summer, and in metro Sydney, the temp was fairly constant. I checked all that you did, and further cleaned the sedimeter, changed fuel filter and looked for leaks in the line. It wasn't till I did a close up check of the line that I found a spot in the fuel line that was worn. When I pulled the line out, rubbing against a chassis mount had worn the pipe away sufficiently to cause a hairline crack. This crack was enough to let air in, and cause fuel to leak back into the tank overnight, but didn't let fuel out, and given that the fuel lift pump was after the crack, no other indication of a problem. In my disco ( My95 -240k) the location was at the drivers side body mount, where the fuel line exits from along the chassis rail, and bends, heading up to the lift pump. It's easist if you run your fingers along the line to feel for irregular bits, as it's a bit hard to see properly.
Good luck
Langy

