Heat oven to 160C/140C fan/gas 3. Tip the potatoes into a large roasting tin with the olive oil, lemon and 100ml water. Put the tin on a shelf under the lamb and cook for 1½ hrs. When the lamb comes out, give the potatoes a stir. Turn up the oven to 220C/200C fan/gas 7 and cook for a further 45 mins-1 hr. They won’t go as crisp as roasties, so poke them with a knife to check they’re cooked through.