I know this is an old thread but I've read quite a few fuel systems/sensor problem threads and no one ever mentions the pre-post heating times or the pre heating unit being responsible for bad starting? I'm not for one minute saying that I have enough experience to tell if this could be relevant in any of these instances but I wonder?
No, the pre-heating is easy to test to see if it is the problem P.G. Check each glow plug resistance which should be between say 0.6 and 1.0 ohm, an if it is way above that it is burnt out and needs replacing. A meter connected to show voltage whilst it is cranking will show if it is getting a signal.
Usually the problem is loss of pressure since these systems work on such huge pressures and any drop will give rise to trouble. So you need a fault diagnosis and a leak off pressure test. Once you have those results you will get an idea where the problem lies.
As I've said elsewhere, as some of the holes are even smaller cross section than a human hair, you only need a tiny spec of dirt to cause a blockage, and then you have insufficient pressure for the system to work.
Obviously if something like a TDC sensor was faulty or the gap too big, that would lead to problems, so that is why the first thing is a diagnostics check. If there are any faults, those need to be attended to.
If there is an internal leak back from an injector, I say replace that one and test again. Only if there IS lots of dirt in the system would I not fit a new one until the system is cleaned out. These injectors are very expensive so you don't want to block a new one.
Roy