In-car versus mains charging

I've been pondering today - is it better to charge my mobile phone in my car, or to plug it into the mains at home? I know I have to unplug it when it's fully charged, as it will otherwise continue to draw power, but is there a difference between the carbon impact of electricity drawn from the national grid and from my car? I feel that somehow the electricity drawn from my car is "free" - but does it affect fuel consumption? And is the effect significant enough to make a difference?

A mischevious colleague has just suggested that I should charge it in my car and, if necessary, take the car for a drive just to provide a charging opportunity. Thanks, Steve, but of course I mean when I'm using the car anyway. In fact, if the impact of charging my phone in the car is lower, maybe it can even help mitigate the carbon impact of driving in the first place. And because I'm bound to be there in the car to hear it beep when the battery is full, I'm likely to unplug it sooner than if it beeped at home when I'm in another room. So which is greener?

Anybody know? I'd love to find out!