faulty injector ?

First off, don't panic (yet)! 200Tdi's a amazingly robust. There's
a lot of things that may have gone wrong - most of them nice and cheap.
At 107,000 miles your engine is barely run in ;-)

The black smoke indicates either incomplete combustion, or over-
fuelling. As you don't say you've done anything to the timing or
injector pump it's probably safe to rule out over-fuelling.

From your comment about noisy tappets, I'm wondering if they have
ever been checked? (Not that anyone ever does!). The valve
stems have little caps on them, which will fall off if the tappets
are slack, and this will lead to the valve timing being completely
wrong, which will mean incorrect compression and black smoke and
rough running. By taking the rocker cover off is should be easy to
see if this has happened. If it has, then 99% of the time new
caps and setting the tappets will fix the problem. Have a look
at the face of the tappets that actually makes contact with
the valve (when I look, I mean as best you can) as this can
become pitted, or even holed in extereme cases.

If the tappets are ok, check all the intake hoses for internal
collapsing - the "lining" can come away and (under vacuum from
the turbo) blocking the air flow (though they look ok at a quick
glance). If its really bad the engine can't breathe and will run
roughly, again with black smoke when things are really bad.

A completely blocked fuel filter/air filter can also give the
symptoms you describe, though they would have to be in a very bad
state, as can blocked fuel lines.

Check the above things, and all the other "simple" things I've
forgotten, and then start to worry a bit - a compression test
would be the next thing to try, if nothing else to rule out
stuck valves, broken rings etc.

If you have lost compression, it may not be the end of the
world, but the spanners may have to come out and then you can
start a mild-to-mid panic!

Richard  

Head gasket?  Again, check compressions.

Martin