Sealing a metal roof

[Isolde1]

Hi, I've just bought a house near Dumfries, that has a metal shed/lean to, attached to the external wall of the house. It's really handy, BUT there's a huge gap between the metal roof, and the wall, so everything inside gets wet (obviously!) when it rains. How can I seal the gap, and make it weatherproof? I've included pics of the "shed" and the gap. Thanks for any advice!