Hi James,
The hub and the bearing are a single, integrated part for the A2. I'd recommend buying a new one.
I believe the Audi units cost about £120, but they're made by third parties... Bosch in this case. Whilst I'm generally one for buying genuine parts, it's possible to buy the Bosch unit without the Audi packaging for about £70.
If you're going to do the job yourself, you'll need a special tool to remove and refit into the knuckle.
The nut that holds the shaft that goes through the bearing is single use, meaning you'll need a replacement. They're cheap as chips from Audi.
Cheers,
Tom