Hi Richard.
Have you removed the engine cover and looked at all the injectors for any pooling around one of them, also do a leak back test, purchase ne of these below.
https://www.ebay.co.uk/itm/6-Cylinde...MK9q:rk:9:pf:0
When was the last time it had a full service, manifold off and cleaned, EGR cleaned or by passed clean MAP sensor, air filter, PCV filter, new oil & filter, etc.
What colour is the smoke, grey/blue does it go away after the engine is fully hot up to temp.
Just a few questions you could ask your self then check and report back to the thread.
PS you do not have a synergy fitted do you ?