I thought I'd clarify this to help anyone thinking of adding the option to their order. I couldn't get much sense from my local Audi dealer when I placed my order.
I believe all A3s come with Hill Assist which holds the car for a few seconds to avoid roll back when moving off from standstill.
Hill Hold Assist is an optional extra and indefinitely holds the car when you come to a standstill. When active (there's a button next to the parking brake switch), every time you stop and firmly press the brake pedal it activates and shows a green handbrake light on the dash. This works particularly well in stop-start traffic, exiting busy motorway slip roads or approaching roundabouts. Just like the parking brake, it auto-releases when you move off again. A light touch of the brake pedal doesn't activate it, meaning you can creep out of junctions etc. if necessary.
It also automatically applies the full parking brake when you remove your seatbelt or switch off the ignition, with a red handbrake indicator lamp on the dash. In fact, it removes the need to use the parking brake switch entirely as it automates the whole process.
When you re-start your car, it remembers the last setting meaning you don't have to re-activate it every time you drive off (if only stop-start would do the same!)
I hope this helps people make an informed decision.