just to be sure, pull vacuum on the out port of the pump to suck some fuel through.. suck a good amount to confirm no bubbles are coming. then try to start.
you can have REALLY bad compression and still be able to start at 0C with good glow plugs. you do need good injectors though... a bad injector or two will definitely make it near impossible to start.
when you start it at low temp, does the motor catch, or sound like it wants to start? or does it just sound like a gas motor without spark?