Oh, by the way, I found out that apparently the Sega Saturn games console uses exactly the same mini-DIN cable as the VW MFD and Seat Navigation System for the video/audio input. Just in case anyone was after a cheap one (they're about a tenner on eBay) with less of the soldering and messing about.
You star! seven squid on fleabay for a fully wired lead, nice one!
As for the firmware, I'll take a look at mine when I get back to the car. If it does start looking like your unit has out of date firmware, then it might be worth heading over to http://vwnavi.com/forum.php. They seem to have a bit of info about upgrading firmware. I don't know the process, and there is a possibility that if you succeed you might be greeted with a VW badge instead of a SEAT one, but it's worth a look.