Okay, I've semi-solved it.
Its clean as a whistle, and has been alcohol'd so that wasn't it.
I tapped it on the back where the jack sits, and it came back to life... I suspect dry/bad soldering to the board to be a cause.
I plug and unplug the socket 2/3 times a day as I use it with my car radio (I do have the official car kit, but the car charger bit broke, and the audio out from the car kit is very bad and also doesn't send my calls through the speakers without disabling loudspeaker while driving !)