When I hear "roast leg of lamb," the first thing that comes to mind is the Alfred Hitchcock Presents episode "Lamb to the Slaughter." It's such a great episode of an amazing show.
I've eaten lamb, but I'm not sure if I've had roast leg of lamb. Roast leg of lamb has been around for so long, I can't find anything about how, when, or where it started. I did, however, find 18th century directions for preparing leg of lamb. If I ever make roast leg of lamb, I'll follow more modern directions. Maybe I'll even use rosemary - that seems to be the popular seasoning for leg of lamb.