My VM dropped smoke in the exhaust on starting, but seemed to run fine once started. Then it got harder to start (same exhaust smoke) until I had to spray WD40 into the instake every time. And then is stopped starting completely.
(1) Glowplugs had never been changed, so I started there - but made no difference (well, it probably did once whatever else was the problem was fixed)
(2) Had it stripped down and the fuel-injection pump came back from inspection as mis-timed. Not quite sure how they can fall out of timing if the mechanical one, but hey...
(3) To add insult to injury, the CPS also crapped out, so replaced that as well.
...it now starts in the workshop, but not put it all back together again to see if it starts on the button after a cold night.