Vauxhall Corsa D cdti cold start problem

I have a 2007 Corsa D cdti 90ps engine Z13DTH, which has the problem of struggling to start the engine when trying to start it when cold after left for 5-6 hours idle , like in the morning's first attempt. When the weather is warmer like now it is a bit easier than the cold winter mornings but the problem is still there. Struggling to start for around 10-15 seconds followed by white smoke. After that the car is working fine, all the rest of the day(no smoke or hard start). Also sometimes although idle for a long time, it fires up immediately but that's very seldom like 1 out of 15 tries.
I have changed & checked:
Battery, Glowplugs, No error codes from scanner.
I am thinking about looking next for: air in my system, camshaft sensor, coolant temperature sensor and measure the pressure. Maybe the problem also could lie in the injector seals or something else, but I am no expert at all. Any other ideas or anyone who had the same problem solved?

Cheers guys!