Bluetooth Car Disconnects :(
So an update for anyone who's stumbled across this question in future;

I think I've managed to solve the problem. Connection restored and music is playing no problem, I've yet to test phone calls and texts yet though.

It would appear the problem was with SYNC and my text messages, I was using the Google hangouts app previously which is notoriously unreliable on older handsets, installed Google Messenger (their new SMS client) this morning, just tried connecting again during lunch and all seems well and stable.

If you're having disconnects especially with a Nexus 4 and (by chance) Ford SYNC, try force stopping, uninstalling updates and ultimately disabling Google hangouts. I'm going to fully remove the app tonight via ADB but for now the disabled state seems fine.