My 300tdi had a problem like this, Just after having a new timing
belt fitted, Ran fine and started ok when warm, However when
cold took along time to fire up, The pump timing was out and it
seems to be quite common for this to happen when a new belt
is fitted.... Even when using the correct tools to lock the flywheel
etc etc..
I put the timing right and it made it start instantly when cold,
I took photo's when doing it and the whole job took less than
1 hour, about 20mins if there is no aircon fitted.
Details of what to do are here:
http://members.mud-club.com/profiles/paul/gallery/timing