Thanks James, it makes sense.
Black smoke can be a number of things from the EGR valve (blanked in your case so not that) through to injectors and the simplest of all - air filter issues. In effect, you are injecting too much diesel in relation to the volume of air intake.
Now it seems you have possibly improved the diesel flow via. a new fuel filter, but how about the air flow via. a new air filter? If it were mine, I would go for the simple option and change it anyway, on the basis that it sets the base line for when to change it again.
What make fuel filter did you fit as a matter of interest, and what make air filter are you going to use if you are going to change it?
Let's hope this will solve it, but if not, you might just be into getting the injectors bench tested, so fingers crossed. Change the air filter and follow Bradley's advice of giving it an Italian Tune, mind it doesn't sound as if it needs it, by your reference to flooring it! I'm set In my ways, but flooring it is a bit like waking you up in the morning, and getting you to run a marathon! OK James, only joking and perhaps enjoy it when you can, because your driving will end up with enjoyment in pace-with-grace.
Let us know how you get on.
Kind regards,
Gareth.
p.s. Can someone enlighten us on VAG's service intervals for fuel filters on diesel engines?