Ford transit smoking alot (when regen i think)

I have a 14 plate Transit which last year came up with service engine now and went into limp mode, took it to my mechanic who said it needed a new dpf as it was blocked and wouldnt allow him to carry out a regen. After getting the van back it started smoking every now and then (smoking very badly) mechanic said it was normal to smoke but when he seen how much he said it didnt look right. He then said the Turbo had play in it so might be causing this, so replaced the Turbo which didnt fix the problem, was then told 4 glow plugs had failed so this might be causing the severe smoke, replaced these which obviously didnt sort the issue. It is still smoking as bad as before but seems to be smoking more often now (not every 300 miles when regen usually takes place)

Does anyone have any idea what my next steps could be or what to look at next? Dont want to keep replacing parts in the hope it fixes it 

cheers