Put the rack at the lowest level in your oven and preheat to gas 4, 180ºC, fan 160ºC. Mix the butter with the sage, garlic and lemon zest. Rub the butter all over the turkey and slice the zested lemons in half and place inside the cavity. Season well. Put the turkey in a roasting tin and pour in the stock or water. Cover loosely with foil and cook for 2½ hours for 5·5kg (11lb) turkey. Remove the foil, increase the heat to gas 6, 200ºC, fan 180ºC and cook for a further 30 minutes. To check if the bird is cooked, insert a thermometer into the thickest part of the thigh; it should read 165-170ºF. Or cut into the leg joint, if the juices run clear it's ready. Transfer to a platter and rest under foil for 30 minutes. Put the chargrilled lemons and fresh herbs around the turkey to serve.