Preheat the oven to Gas 9, 240°C, fan 220°C.
Remove any excess fat from the lamb. Make incisions in the lamb skin and push a garlic clove into each one. Rub the lamb with salt and oil and scatter with rosemary.
Place the potatoes and carrots in a large baking pan. Place the lamb on top. Cover with foil. Roast the lamb in the hot oven for 10 minutes before turning the heat down to Gas 1, 140°C, 120°C for 2 hours.
Turn the potatoes and carrots, spooning any fat over the lamb before closing the foil again and returning to the oven for 2 hours more.
Remove the lamb and vegetables to a serving platter while you make the gravy.
Spoon off the excess fat. Strain the gravy into a pan. Whisk the flour into two spoonfuls of chicken stock before stirring it into the pan with the lamb cooking juices. Mix well and bring to the boil on the hob. Add the capers, anchovies and mint sauce and boil together for 3 to 4 minutes. Slice the lamb and serve with the vegetables.
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