I'm no expert by any means, but there are really only about 3 categories of problems with regard to starting, and this really is for any vehicle. You have to have fuel, fire/ a source of combustion, and a source of ignition.
1. It could be an issue with fuel delivery. (e.g. Injector pump, injectors). With normal outdoor temps, white smoke may indicate a timing problem with your injector pump. 'Course, when it's cold, mine always gives a little kick of white smoke on start up. You may have a bad injector or two, but it doesn't really sound like it.
2. It could be a "fire" problem, that is, when it's cold, the compression of the engine doesn't raise the air temperature enough to cause combustion. This is what the glow plugs are for. This is really what it sounds like is going on. You already know you had GPR issues, so I'd check the plugs again, even though they're only 3 mos old. Check that the relay is good. If the plugs and the relay are good, but there's a problem, it may be the control card. Pretty sure flmv.net has these for a reasonable price.
3. Finally, it could be an ignition problem (bad batteries, alternators, starter, etc). If the batteries aren't carrying enough charge due to the cold, they may not provide enough power to the starter to turn the engine over and start the combustion cycle. Be sure your batteries are fully charged and tested. Check that you have full power to the starter. Of course, you said that it seems to be turning over, just not catching. So I don't think it's this either.
Really though, I think the demon is hiding in your glow plug system somewhere. Time to get out the multimeter and go hunting.
