2. Roast for 30 minutes at 220°C (Gas Mark 7). 3. Reduce oven temperature to 160 °C (Gas Mark 3) and continue to roast for: 1  Rare – 20 minutes per Kg (9 minutes per pound).oast for 30 minutes at 220°C (Gas Mark 7). 3. Reduce oven temperature to 160 °C (Gas Mark 3) and continue to roast for 50 minutes per Kg (23 minutes per pound). 4. Remove the joint from the oven and check that juices run clear. 5.