I've been looking into this, as I've just ordered my GTI and refuse to pay full price for the Satnav! Dj.Kolia on e-bay seems to be a preferred source according to what I've found on various forums, and the latest 'C' model (£499) allows you to play a CD as well as navigate - beware, I don't believe that earlier versions do... He reports to be UK-based, too, which would provide me with a bit of reassurance.
I'm also tempted due to the reported increase in audio quality, although I wonder whether this might be dictated by the number of speakers - RCD300 owners will only have 8 installed, whereas if we'd specififed a 500 or MFD 2 then we'd have 10........