How bad is this? It's only sometimes in first and second gear.

Could 'just' be injectors or timing, but could be something more serious like water getting in i.e. head gasket 

all the time, or just when it's cold ?

white smoke = un-burnt diesel

pretty normal if the engine is cold :-)

White smoke is usually just steam.
Unburnt diesel is black.

More than likely just a bit of water in the fuel. Check if your filter has a water drainas they can get full.

I doubt it is coolant as that stinks really bad when it is burnt.

un-burnt diesel is white !

black smoke = bad fuel mix, not enough air so loads of soot

water in the fuel wouldn't be enough to make white smoke/steam, only a tiny tiny amount of diesel is injected on each stroke, even if it was 50/50 diesel/water it wouldn't be enough so you could see it

blown head gasket can mean water into the cylinders and then steam out the exhaust... but normally there'd be clouds and clouds of it, and exhaust gas up into the header tank too


if it's just when the engine is cold ignore it (unless there's clouds and clouds of it)

how old is the engine / how many miles ?
(what kind of car too?)

It means they've elected a new pope.