Preheat the oven to 180C/160C Fan/Gas 4. Sit the lamb in a large roasting tin, skin-side up, and use the tip of a sharp knife to make many incisions all over the skin.
Put the oregano, rosemary, garlic infused oil, lemon and orange zest, 2 tablespoons of lemon juice, 2 tablespoons orange juice, olive oil and salt in a bowl and blend using a stick blender (alternatively, use a small food processor).
Spoon this over the lamb and rub in well. Leave at room temperature for 45 minutes. Pour ½cm freshly boiled water into the bottom of the roasting tin and roast for 1 hour 40 minutes.
Check after an hour to add more water if needed and cover with foil if the meat is beginning to catch. Just before the time is up, remove the lamb from the oven and check it is done to your liking, either using a probe and the temperatures listed in the tips below or piercing with a knife and taking a peek.
Remove the cooked lamb from the oven, cover loosely with foil, and rest for 15–30 minutes, checking every now and again, before transferring to a board to carve.