So,
I can report back that I've managed to get it all working, but, don't get too excited, functionality is a tad limited as it stands right now.
Android Auto - Even though my phone had AA installed, I installed another copy from Google Play. Plugged it in and eventually you can add the phone and you get Google Maps navigation with voice input even if you don't have that option in the car. You get 90 days of Google Music, just make sure you've got enough data on your mob to cover it... Erm, and that's about it.
Mirrorlink - I must admit I was expecting something close to a 'mirror' of what was on the phone, however, so far all I see is the icons for Mirrorlink enabled apps which in my case are the various VW ones - mind you it does have the journey tracker which records stuff like how hard you accelerate, brake, max speed, G force and what have you - which looks about the most fun one.
Switching between AA and Mirrorlink isn't particularly simple. It appears that you have to go into the settings change the preferred connection type between the two, then disconnect and reconnect the device - I haven't found a simpler way yet.
Finally car-net, I'm now registered and functional. The process is straightforward, however, it fell over several times and was really slow.
The link for registration is as follows:
https://www.volkswagen-car-net.com/portal/en_GB/web/gb/register?command=cEventually it will give you a code. Get the car connected to the web by tethering to your phone or what have you and enter the code under Menu | Car-Net | Registration - this bit took numerous attempts but my mobile signal was very weak it may be better for you guys.
I've got Discover Nav so I get weather, news (you can add things like BBC news, sport, etc, as feeds via the car-net portal), parking, petrol, online POI, personal POI import, etc. Not loads but a few things. I'm sure we'll get more in the future.
Apologies for the essay!
Cheers, Neil.