Has just happened to my A3 too - just over 1 year old! The weather was nice and sunny, so put aircon on Lo, it seemed quite cold to start with, then after about 15 miles it really was blowing out warm air. Rang local dealer, they again said to me that they would not be able to tell me if the problem was covered by warranty until they had taken a look. Took it in last Thursday and told that I needed a new aircon compressor, do not have any in stock, 10 day delivery from Germany, and is covered under the warranty at a price of £540 for the part and £60 labour (2hrs I was told). So after the 23rd May mine should be working again!