Hyundai Santa Fe (2003) Refuses To Start?

I would be *very* appreciative of advice with my Hyundai Santa Fe refusing to start please: Its an 03 plate, manual, diesel, GSi CRTD. There doesn't seem to be a pattern to it not starting (although the probability seems to be higher after a very cold night): One of the glow plugs was found to have melted (to the shape of the cylinder head ~ which Hyundai didn't care about) and they were changed - the car started for a week then stopped starting; Next one of the injectors was found to be faulty on a back-leakage test and was changed - the car started for a week then stopped starting; Next the seals on the pressure regulator were replaced and the crankshaft sensor connector cleaned - the car started for about 3 days then stopped starting; Next the fuel pressure was found to be weak and the OEM fuel filter was found to be defective (which Hyundai admitted was a known issue with the Kia Sorento and yet they didn't care about it) and full of dirt so the fuel filter was replaced and the tank drained and cleaned - the car started for 1 day then stopped starting. This is where I am now, utterly confused as are the 2 garages I have used (one a diesel specialist) ....?