A bodyshop will charge you £650 - £800 to do it properly, a friend of mine scraped her car in a parking accident, door respray was £700. Insurance paid up, she thought of covering the cost herself but when she heard the price changed her mind.
EDIT: not just door, also rear wheel arch resprayed.
If you have no claims protection on your insurance you should consider letting the insurance deal with it, plus there's the issue of the corrosion warranty with a new car. Get it done properly, would be a shame to be the driver of a 3 year old Bravo with a rusty door.