How can I re-attach lead panel that has fallen off roof?

A five foot long lead panel has come off my porch roof. It should sit at the top of the roof above the door, this part of the roof is attached on to the outside wall of the house just above the front door, forming a small porch. The lead is undamaged, I have had a look at where it came from and there is a horizontal slot along the outside wall for it to fit into, however it doesn't look like it was ever attached properly. What's the best way to re-attach the panel so that it doesn't come off again?