Another A4 Fault Code Question

Hi all,

I know there are plenty of these questions floating around but I have yet to find one which seems to be suffering the same symptoms as me. So here is goes, hopefully I will cover everything needed for you guys to help me out.

The other day about 3/4 of the way through the journey on my way to work (a 13 mile journey) I started getting a surging output through the 1000-2000rpm range coupled with thick output from the exhaust.
I made it to work and the response from my dad was to give it a bit of a low gear thrash to see if it was just a dirty turbo. I did whacked it to 60mph in 3rd which spat out a pretty dense cloud of dark exhaust and after that point I had lost the turbo and was in limp mode.
When I got home I plugged into VAGCOM which threw up the 17964 fault code (negative deviation on charge pressure) which I tried clearing and took the car back out for a drive to find the turbo had returned and the surging didn't appear to be back.
However this morning I noticed I had 1st and 2nd gear surge, again coupled with black smog out the back when the surge hit, and then lost the turbo again when I tried to accelerate hard. Rinse and repeat for the journey home.
I noticed that when the surge hits there is a sound not dissimilar to missing a gear/trying to change gear without the clutch fully depressed.
I have also noticed that (to me at least) there appears to be a lot of noise of rushing air/engine noise from the passenger side when the surge is hitting.

I know that the fault code above tends to relate to vacuum leaks and/or N75 valve failure and that a characteristic of one/both of these is the turbo returning up at 3000+ revs; which isn't happening.
I don't know if it is relevant or not but at the weekend I had to drop off a hardstand and I bottomed something out while doing so. It felt like it was just behind the pedals. I am concerned that in doing so I may have busted the exhaust which could be causing a leak which is affecting the turbo.
For reference I have a '52 plate Audi A4 estate (B6 variant?) 1.9TDi 130BHP engine. I forgot to get the engine code of VAGCOM but I will do that over the weekend.
I only have VCDS Lite fyi for any suggestions to try.

Thanks in advance I know it's a long lead in but I though all info could be useful/relevant.