any idea if it's worth using oil additives before an MOT as well? i reckon if it can pass emissions, it's worth doing. if not, I'll break/scrap it.
Engine oils already have additives in them to do the job in hand, if you see any 'detergent' type additives on the shelf keep on walking past as your oil is already doing cleaning in the 1st place (as long as you bought some decent oil).
Personally, if the car is producing smoke and there is no mechanical fault with the engine then its most likely the fuel thats slightly dirty.
Get a new fuel filter (£17) and refill using Shell Diesel V-Power (costly but very very clean fuel), find a long hill you can drive up and make the engine really hot. Blast all those cobwebs out!!
Millers Diesel Sport 4 will also reduce smoke, but again I doubt if you would see any improvments until half/full tank run through.