First off, thanks to all those on this forum who answered my queries re. latest version numbers and p3eps' how-to. Your help basically gave me the confidence to go for it and I'm glad I did.
Buying: I sourced a seller in Lithuania (yes, I know...) on UK eBay with a solid profile and ordered the latest LED / 6.1 software version from him at lunchtime yesterday. It was £550 + £25 shipping. Amazingly, the parcel arrived this morning, less than 24 hours after I payed
Opening the parcel: it was extremely well packed with thick slabs of polystyrene protecting the unit, which itself was wrapped in several layers of cellophane. In addition to the headunit, there was a DVD and a GPS antenna. Packaging was not OEM, however, so I can only speculate as the exact origin of the headunit. There was an 'official' VW card with the security code on it, however.
Installing: this is always the scary bit, but p3eps' excellent write-up here
http://www.golfgtiforum.co.uk/index.php?topic=152441.0 made it look simple and indeed it is. I think 15 minutes is a bit optimistic: it took me about 30 minutes all together, but that's probably because I'm thick
Setup: amazingly, it really is plug & play. Everything worked immediately, including the Climatronic / heated seat display, parking sensors and steering wheel controls. The only moment of slight panic was when it didn't seem to be picking up the satellite signals. Current position seemed to be stuck on a street in Germany, despite the setup menu saying it was detecting 5/10 satellites. It did eventually realise I was in France, though, and now seems to be working fine.
I haven't tested all the features yet, but I couldn't help putting in a DVD movie (Die Hard 4), just to see what it's like. Fantastic, especially as you get almost home cinema quality sound through all the car's speakers. The screen is very bright and the whole system seems significantly faster than my 'old' RCD510. Also, the sound quality (radio) seems very slightly better, though this could be just my imagination.
I am selling my RCD510 + TomTom and hopefully should get about £400 for both, which means the RNS system should only end up costing me about £175. Well worth it, IMHO.
Thanks again to all of you who helped.
The Frog