Hi
Having completed a retro armrest mod to my first A1 I can say it's a reasonably easy job as long as you follow a few guidelines in the link below.
I would recommend buying the genuine Audi armrest and not to bother with the eBay cheap imports. Stoke Audi Parts were always very helpful and will deliver.
The instructions below are worth reading. One extra piece of advice is to tape each screw to the screwdriver when you come to secure the main armrest bracket to the floor pan. If you don't and they fall off and drop down the gap, you'll never find them.
http://www.a1-forum.co.uk/a1forum/viewtopic.php?t=2324
Good luck
