May i offer a view on this one. I would take out the lamb from the fridge an hour before cooking. Skewer the meat as much as you can then If you have a frying or sauce pan big enough seal the lamb and put on a grill rack or a cooling rack or chop up some carrots and place the lamb on it inside the roasting tray. As long as the lamb is elevated your lamb will lose it's fat better and give you a better chance of avoiding to fatty a lamb. wearing my Chef's hat I would not trim the fat off before cooking as the fat helps to distribute flavour. With my dieting hat on i would trim as much fat as you can off the beast, If you can find sprigs of fresh rosemary slice a neat cut into the lamb and stuff the herb into it. When it is cooked leave on the rack in the tin on your work top for at least half an hour before carving.
hope that it works for you.