Emissions Issue

[Somersetting]

Hi Peeps, MOT failure on emissions and I can see why as it puffs a little dirt on acceleration but no visible smoke when running or so I am told, however had the Injectors cleaned and reset mapped etc, with new Air filter fuel filter and EGR valve cleaned but (seems to have been replaced by previous owner). The DPF is clear so I am led to believe. The Wonderful mechanic who is trying to fix says that it looks like the timing cover on the front has been removed (Squidgy sealant used) and he eludes that if the Timing is just a fraction out this could cure the excess smoke. I do not doubt him but unfortunately he cannot continue with the works as My original request was slipped in between other busy works. So any ideas on the timing issue? would this clear up the puffs? Is there something more sinister that you guys may know of.
Would appreciate a few ideas to mull over.