You might still catch HH's spring deal bud - give them a call tomorrow...
I've never tried it myself but a lot of the guys on here including Monksy swore by 2 stroke oil. I would try a bit of that with every tankful of diesel mate.
If the smoke is the result of the tune then you aren't going to get rid of it with any additives - there might be a small element of "burning out years of muck" but really the smoke is just the way the engine is burning (or not completely burning) the fuel. There are no "smoke pixies" that you can banish.I'm just stressing out as I have a MOT due in a couple of months.
Well I'm always surprised when mine passes!LOL you sound surprised.