It was replaced under warranty, if it wasn't covered I think vw were saying about £700, but I think an independent would be a lot cheaper!
I think when you start putting a bit more power through the manifold the flap struggles and ends up being stuck, but apparently it's all one piece so you can't just replace the flap you have change it all, don't know how true all this it's just what the dealers said but I'm sure some people on here will no a lot more about this! Try google and you can see that it is quite common and good luck.