A8 misfire

[Evilcclive]

New to this, so here goes!
I have a Audi A8 3.0 TDI ,I'll start from beginning.
My inlet manifold was blocked got one cleaned out also the egr valve.
For a few weeks after I was getting a engine management light on saying breathing issues.
Cleared these everything was fine until last week.
Driving home from work the car started spluttering, took it to my mechanic who diagnosed a injector fault in cylinder 5.
Got it replaced and recoded, but still the same problem.
I took it to my friend the auto electrician who went through everything on the car and found no errors at all.
But still misfiring in cylinder 5.
I'm also getting white smoke out of the exhaust.

I've also noticed a sooty residue near the turbo, I know the turbo is ok because it was changed six months ago.

Also now when I take my foot of gas peddle I'm getting a funny noise .
At speed car pulls fine.
When ticking over it seems like it's ticking over too slow. No problems starting charging ok.

I've been trawling the net for answers but nothing seems the same and people with silly basic replys.
Any help would be very gratefully received!