Hi 'sallabananas' - Somewhere in your handbook it should tell you the approximate mileage left when then light comes on, although I suspect they will be rather conservative with their estimate.
I was always told not to let the fuel level go too low in case it brings up sediment from the bottom of the tank. Also, I wont buy petrol if I see the delivery tanker on the forecourt, again because I heard it disturbs any sediment in the storage tanks.
Do you think I'm being paranoid about this??