There is, as you suggest, an Audi accessory, specially designed to carry bikes on the back of the A2. If you have a rack though, you might as well just get a bike adapter kit - it'll prove more versatile in the long run.
There are loads of posts and pictures of bike racks in the archives. To access, click on Search in the top tool bar and type 'bike' in the drop down box. For more posts type in 'Thule'
