reduce the oven temperature to 150°c, gas mark 2 and roast for 1¾–2 hours, until the lamb is really tender.
5. about 20 minutes before the end of the cooking time for the lamb, boil the potatoes in a pan of lightly salted water for 10–15 minutes until they are soft around the edges. drain in a colander and shake them a little bit to rough up the edges even more. transfer to a roasting pan and spray with oil, tucking the rosemary in between them.
6. remove the lamb from the oven and leave to rest for 30 minutes. turn up the oven temperature to 220°c, gas mark 7. roast the 