Preheat oven to 220°C. Remove excess fat from chicken cavity.
Place lemon and thyme in chicken cavity. Combine butter and oil in a small bowl. Brush over both sides chicken. Place, breast side down, on a greased roasting rack in pan. Roast for 35 minutes.
Meanwhile, place potatoes in a saucepan and cover with cold water. Bring to the boil over high heat. Cook for 10 minutes or until almost tender. Drain. Return potatoes to saucepan.
Turn over and baste with butter mixture. Arrange potatoes around chicken. Brush with butter mixture.
Roast both for a further 37 minutes. Cover and stand for 15 minutes to rest.