Ya know, the best thing i invested in (you can get them at bigw, kmart.. probably even woolies or coles) is a meat temperature gage. You just stick it in the meat and it has a dial on it that will tell you when its cooked in the middle. I put all my roasts in on 180c. Ive gone on the so much time per 500gs before and my roasts have always come out undercooked in the middle. But with the gage you never ever have that problem! They dont cost much.. like $7 maybe. Best thing to have in the kitchen!
With the lamb roasts.. i just rub them with some garlic salt and rosemary put it in an oven proof dish and they come out sooooo yummy!! Thats it.. nothing else to it. My roasts have never been dry ever doing it this way.

Best of luck!