Place the garlic, anchovies, leaves from 6 of the rosemary sprigs, olive oil and black pepper in a mini processor and whiz to combine. You can do this with a mortar and pestle if you prefer – mix the garlic and anchovies first, then chop the rosemary leaves finely and add them in along with the olive oil and pepper.
Place the lamb in a large non-metallic dish and rub the mixture all over it. Cover with clingfilm and set aside for 2 hours to allow the flavours to combine. Overnight in the fridge is even better.
Preheat the oven to Gas Mark 7, 220°C (450°F). If the lamb has been chilled overnight, bring it back to room temperature. Then place it skin side up, in a large roasting tin. Scatter the reserved rosemary sprigs over the top. Roast the lamb for 10 minutes, then turn reduce the temperature to Gas Mark 4, 180°C (350°F) and continue to roast for another 35 minutes. The lamb will be cooked medium at this stage. Remove from the oven, transfer to a carving board, cover loosely with foil and allow to rest for 10 minutes before carving.
Add a dash of wine or water to the juices left in the pan and boil.