1. Preheat the oven to 220°C, gas mark 7. Combine the pepper, garlic, capers and rosemary in a small bowl. Push roughly half the garlic mixture into the boned cavity of the lamb, so it’s fairly evenly distributed. Rub the remainder over the outside and place the lamb in a medium roasting tin. Roast for 30 minutes.
2. Meanwhile, make the sauce. Tear the pitta bread into pieces and put in a bowl with the milk. Leave to stand for 10 minutes until the bread has softened. Tip into a food processor, including the unabsorbed milk, and add the garlic and anchovies. Blend until completely smooth, scraping any lumps from the sides of the bowl. With the machine running, slowly trickle in the oil until the sauce is thickened. Season if necessary. Add the vinegar and transfer to a serving dish.
3. Reduce the oven temperature to 180°C, gas mark 4 and roast the lamb for a further 1 hour (see tip below). Scatter the tomatoes, olives and basil around the lamb, turning them in the pan juices. Return to the oven for 10 minutes. Meanwhile, place the potatoes into a separate tin in a single layer and drizzle with the oil. Season. Roast on the shelf above the meat for 45 minutes, shaking the pan occasionally, until the potatoes are tender and golden.
4. Lift out the meat, tomatoes and olives onto a board or platter, cover with foil and leave to rest for 15-20 minutes. Add the wine and stock to the roasting tin with a little seasoning, and bring to the boil over a high heat, scraping up all the pan juices. Cook for 2 minutes until slightly reduced. Carve the lamb and serve with the tomatoes, garlic bread sauce, gravy and seasonal green vegetables.
Cook’s tip For lamb that’s cooked right through, rather than pink in the centre, increase the cooking time at the lower temperature to 1 hour 45 minutes.