Cold weather starting

Sounds like a glow plug issue from your description,also check your engine earth lead to chassis is good.

I reckon you have a fuel pressure problem.Do you use supermarket fuel?

Ok 2 different ideas as to what's wrong ,I would not discount either suggestions as they should be on your basic checks list, Also has your fuel filter been replaced as maybe water in it icing at low temp, Just we have had 2 cars with this issue this week possibly due to -6 temp.Are there no fault codes logged,to guide diagnostics.Other thing totally outside the normal are the correct glow plugs fitted ,as I had an issue where the wrong ones were fitted and only when it got cold ,because they were not protruding into the cylinders properly ,the van would not start,worth a check,let,s know how you get on.

I had the same problem earlier this year with my C5, everyhing was pointing at a fault with the turbo system and mainly to the turbo boost pressure solenoid! After lenghthy investigation i found it to be the air flow meter, an easy way for you to check this is start the car and whilst it is running disconnect the air flow meter, drive the car and see if it drives as it should if it does then a new meter is req. If you disconnect the meter with the engine stopped it will still be the same sluggish self as before. Good luck, i hope this works for you.

Sorry i am giving the right info but to the wrong person with a different problem.... I meant to write that i have the same problem as you now and have discovered this evening that my glow plugs are not energising on ignition but do energise when the engine is running at idle speed, when you up the revs they turn off then back on again when you release the accelerator pedal. I will be testing the cicuits tomorrow and checking the relay, i will let you know what happens and then you may have an idea at where to start.