Preheat the oven to 220°C, fan 200°C, gas 7. Weigh the lamb leg and calculate the cooking time. Allow 18 minutes per 500g, plus 15 minutes for medium lamb, or 20-22 minutes per 500g plus 15 minutes for well-done lamb. Rub the olive oil all over the leg, season, and put on a rack in a roasting tin.
Pour the wine or stock around the lamb in the base of the roasting tin. Roast for 15 minutes then reduce the temperature to 200°C, fan 180°C, gas 6, and continue roasting for the remaining calculated time.
Baste the lamb frequently, adding a little water to the tin if the juices start to dry up. Use a meat thermometer inserted into the thickest part of the lamb to test if it’s done. For medium, it should read 70°C, and for well done, 80°C. If you don’t have a meat thermometer, a metal skewer works well. Insert into the thickest part of the lamb and leave for 30 seconds. Pull it out and touch the tip against the inside of your wrist. If it is cold, the meat is not yet done. When it’s warm, the meat is rare, when fairly hot, it’s medium, and when it’s very hot, the lamb is well done.
When the lamb’s done to your liking, transfer it to a warm platter, cover with foil and let rest for 20 minutes before carving. Skim off and discard any fat from the juices in the bottom of the roasting tin, then pour the juices into a warm small jug to serve as a thin gravy.