I know small boat owners have a massive problem with mildew when a boat has been unventilated for some time and I asked a friend what he uses. He uses Nosguard Star Bright Boat Bomb. As I understand it, its actually designed to remove the odours associated with mildew but it has quite a succesful side that it seems to kill mildew too.
I have a very large shed and I bought some shelving from Big Dug ( Fleabay). Over the winter, some of the hardboard shelves became covered in mildew, however none of the real timber, i.e. shed, workbench, flooring were affected. After removing and disposing of the hardboard shelves (ALL 20 OF THEM) I tried some of this and have had neither spread nor repeat occurence.
See http://www.marathonleisure.co.uk/product.php?xProd=224960
[video]https://www.youtube.com/watch?v=U6FlIMwWEcM[/video]
Hope this is of some help.