AbraBennet has a mocha lamb recipe that is delicious:
http://www.food52.com/recipes/3778_dark_mocha_lamb

I'm not crazy about lamb sirloin. I would be tempted to grind it up and use the meat for shepard's pie or even kibbeh. As a note, I live in lamb country and the idea of using imported lamb from the antipodes is appalling.

Here's a Food & Wine recipe for Grilled Lamb Sirloin: http://www.foodandwine.com/recipes/grilled-lamb-sirloin-with-greek-salad