Talking to myself (again)
I've found something called an aircast:
http://www.amazon.co.uk/Belkin-F4U0...YQZ8/ref=sr_1_1?ie=UTF8&qid=1339849436&sr=8-1
Plugs in to the aux socket in the car and connects to the phone over bluetooth, lets you take calls as well as streams music from the device.If a call comes in it can pause the music and resume after. Uses A2DP for the music and I assume AVRC to skip tracks/pause etc (the phone supports both standards), so i guess it should work.
Alternatively they also do this: http://www.amazon.co.uk/Belkin-F8Z4...HUSE/ref=sr_1_5?ie=UTF8&qid=1339849200&sr=8-5
Which is basically a bluetooth receiver. There must be rival products, not found them yet.
At the risk of boring everyone I received the aircast today and it works fine with the phone, pairs up really easy and almost all functions work (take call, play/pause music, skip track). The only one I didn't get working is activating voice dialling- the aircast says it will activate voice dialling but the S3 didn't respond. I did wonder if it would enable S-Voice but it appears not - mind you I do have double tap on the home button disabled.
Audio quality is fine both on call and music.
Cheap way to use an aux in socket and get bluetooth calling.