Pre-heat the oven to 200°C.
Open up the leg of lamb so that it is butterflied. Rub the inside with the butter then season generously. Place a mixture of rosemary and thyme sprigs in the centre and roll up the sides of the lamb around the herbs. Tie securely at 2 inch intervals with kitchen twine.
Arrange the potatoes in the base of a roasting dish. Drizzle over the olive oil and some seasoning and toss to mix well. Sit the leg of lamb on top of the wedges and arrange any remaining thyme sprigs around the lamb and on top of the wedges.
Roast for 15 minutes then reduce the temperature to 170°C and bake for a further 45-60 minutes depending on desired cooking degree of the lamb. Remove when ready and allow to stand covered with aluminium foil for 10 minutes before serving.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.