If you're lucky and they can get at the panels from the inside to beat out the dent then it should by £350~400ish judging by work I had done to one of my front doors.
The problem is likely the rear panel. If you've managed to get the dent in an inacessible part (due to body stiffening etc for the roof) then you'll be straight into £700+ territory in my experience.
Good luck and let us know how you get on.
PS: Welcome to the forum!