I have been reading this and feel for you Rob I really do.
But I have to point out what seemed obvious to me from early on....I just dont think you've taken your car to the right places. AB tech, well you have a different opinion of them now....but then to go to Audi.......£100 per hour for guys who bolt brake pads in and change oil etc........to investigate a fault of the nature you have.......sorry, but I think you need a specialist place that knows these motors inside out, not an Audi dealer who's main priority is their bottom line and for the techs, their bonus!!
I very much doubt they will diagnose the problem, more likely start randomly changing things and charge you for all the things that dont work.
A GOOD specialist will charge £50 per hour, have more knowledge about suhc things and be more motivated to sort it out....Southampton Audi dont care if they dont find the problem and you let everyone know they cannot like Abtech could not...?!?? A specialist is surely the way to go with this, also you are tryign to get Audi find something that you can pin on Abtech....they have the ultimate get out clause for not being ablee to sort the problem.....?
Now I dont know who to recommend as such, but I would suggest, depending on the status of the car itself now, ie running, not running, running badly, drivable etc, that if its drivable you take it to someone that can run some professional vag come logs on it whilst on the move....... Thsi should highlight your problem more accurately than "its getting too much air....." (what does that even mean.....!??!? At idle, under load, too weak a mixture what....?).
In my slightly cynical opinion, you wont get too far at Audi, they just dont knwo enough about what are pretty old cars now.
If vagcome and logging cannot pinpoint the problem, you then need to think about having someone strip the motor out, and chekc every single pipe, valve, wire etc etc for damage splits etc.....?
Or burn it for the insurance! !
Sorry Rob,m for what you are going through and you are doing the hand deep in pocket thing with supposed experts, but IMO you may have been better to speak with other people about this.....?!
good luck with it