you can change 2 lambdas that you can see if you look down the side of each bank, you will more than likely find that a bottle of cataclean will fix this and the light will turn itself off after around 150 miles or plenty of start stops after a long run to clear it out, bank one is driver side, they are not to bad to change and i found an ebay shop that dose parts cheap to, lambdas are about 80 quid each
also did the W12 that A8 parts have not get broken because of the lambda and cats failing ? and the lambdas are on ebay for sale ?