Hi All i hope you can help

i have a 2008 vectra 1.9 cdti 120

it is hard to start and when it does it will give out white smoke, engine will tick over all day long with no problems, it will pick up fine, i have had the glow plugs changed and the crank shaft sensor as the spanner light is on and giving this as the fault. tried 3 different sensors but to no avail. car has just been serviced.

any help much apriciated as i am running out of ideas