One or two days before serving, trim lamb of fat to the degree you like and score fat in a criss-cross fashion. Combine yoghurt, honey, salt, rosemary and garlic. Rub into lamb, then cover and place in the fridge.
A few hours before the meal, take lamb from the fridge, bring up to room temperature and wipe off excess marinade.
An hour before the meal, turn oven to 220°C/450°F/ Gas Mark 6. Heat up a heavy frying pan and add a few teaspoons of oil. Place as many racks as fit comfortably into the pan, fat-side down, and cook over a moderate heat to render some of the fat, then turn over and cook a further 2 minutes. Transfer to a roasting dish. Place in the oven near the top and roast 15 minutes for medium rare, 20 minutes for medium. Rest in a warm place for at least 10 minutes before serving.