They can get all the codes they need from your car and by pulling the stereo out. I got it done for free from VW in Ashford, which I proptly lost and then when I phoned VW in Canterbury they wanted about £20 for it.
Luckily I took it out myself, unplugged everything left it for 5 minutes and then put it all back together again and it worked. When I turned it back on it said LEARNING on the screen and then a few seconds later it started working.