smoking astra!!?

1.7 mk 4 2004 astra van cdti..high mileage!
Bought the van a couple of weeks ago and it has been running quite rough since..had a look at the fuel filter which needed a change..since then i have changed the filter 3 times due to the muck on it..used diesel cleaner and it seemed to improve for a while but now getting white smoke when pulling away or accelerating..not all the time mind..engine seems to miss, puffs a bit of smoke and clears..runs rough at low revs ie second gear in traffic seems to "hunt" as it used to be called! and a couple of times when pulling away at low revs it has stalled and been hard to start...thinking injectors anyway of testing without taking to a specialist??
sorry about the length of this but like most my van is my living!!! thanks in advance