Welcome to the forum Simon, you'll find the members on here are a friendly and helpful bunch :)
I can fully relate with what you're saying as my 2002 a6 does the same to me! As does my old Ford escort. Bills are expensive!!
Now there are 2 ways of looking at this:
1. With your head. Is it time to get rid as it's caused too much grief. Or can you now not afford to sell in light of how money has been spent and you're now stuck with it.
2. With your heart. In my opinion if it puts a smile on your face every time you drive it then is the expense worth it. I tell you now, despite how much money my Ford escort costs me. I'd never get rid of it due to it putting a smile on my face! Hope that makes sense.
It sounds like to me too many owners have neglected servicing and maintenance on the car and now you're the one putting it right. Once it's sorted you'll have a cracking car. And if you love it then I'd say persevere. I've found that expensive bills get more common the older the Audi gets. But this is the same case with any car. And I love my a6 and wouldn't part with it.
Cheers
Steve