How about using a Ring MP75 Miniverter? This will convert your cigarette lighter into a 240v power supply - should be fine for a Canon camera. Only £24.99 inc delivery too - I use mine with a photo printer in the car Link
Thanks Adam for the heads up on the CR560 But its very pricey solution so will go for Larrys solution of the the Ring inverter as it has other uses as well.