hi mate my 2006 gti aircon stopped working last month so took it to my local aircon service fella for him to take a look he plugged it in and checked the system which was fine and still had 90% gass and oil levels even after 3 years and 40,000 miles so took it back to dealers as its just about still under warranty and had a big row with the because they were saying the first thing that they wanted to do was a full aircon service that would cost me over £100 obviously i went mad and demended that they atleast check it all over before trying to charge me as i told them it had already been checked for gas levels etc, Anyway after they agreed to look at it first it turned out that the compressor had blown up (a common problem apparantly) luckily it was all covered under warranty as took them 5 hours to sort it out after ordering the new compressor which cost £700 plus labour charges (joke) anyway it all tuned out good in end as they serviced whole system and re-gassed etc which saved me money as was due a aircon service next april (4 years old)
If it is just that you have ran low on gas etc a refil at local place is usualy between £45 and £55 hope this helps mate