Preheat the oven to Gas Mark 6, 200°C (400°F).
Place the chicken in a roasting tin and put the lemon and a sprig of thyme inside the cavity of the chicken. Strip the leaves from the remaining thyme stalks and place in a bowl along with the garlic, olive oil and seasoning. Mix to combine, then brush this mixture over the chicken.
Cover the chicken loosely with foil and roast in the oven for 30 minutes. Then reduce the temperature to Gas Mark 4, 180°C (350°F). Remove the foil and roast for another 50-60 minutes. It's fully cooked when the bird is pierced in the tigh with a skewer and the juices run clear.
Remove the chicken to a carving board and cover loosely with foil. Allow to rest for 15 minutes while you make the gravy.
Stir the pan juices over the hob to heat through. Add a little stock or water and boil up.