With a knife, follow the lamb bone down about 4 inches (you are just making a 'tunnel' where you are cutting the meat away from the bone). Do this from the top end and the bottom end. Pierce the skin at an angle with a sharp pointed knife 6 to 8 times at random around the leg. Where you have made the incisions, open them up a bit by poking your fingers down in them to make a bit of space.
Using a pestle and mortar smash up half the sage with the garlic and 1 teaspoon of salt. When its pulped, add the lemon juice, 2 tablespoons of olive oil, the other half of the sage and the rosemary roughly chopped. Stuff the mixture into all the incisions and gaps you have made with the knife. Then stuff the pancetta deep into the gaps along the bone and the incisions. This gives a lovely fragrant flavor to the meat. Season with salt and place directly on oven shelf in a pre-heated oven 225 C/425 F/Gas 7 until cooked.