Preheat the oven to 220ºC, fan 200ºC, gas 7. Rub the lamb all over with the oil and a little black pepper. Put in a roasting tin with the garlic and rosemary.
Roast for 20 minutes then turn down the heat to 190ºC, fan 170ºC, gas 5 and roast for a further 1½ hours. Remove from the oven and let rest for 10 minutes before taking out the bone (as directed on pack instructions).
Carve 150g lamb per person (cool the rest, wrap in clingfilm and store in the fridge for up to 48 hours to use in other meals).
Serve with a selection of your favourite vegetables and mint sauce or gravy.