Mix the buttermilk and finely chopped rosemary together in a large wide bowl. Add the chicken and turn so that everything is well coated. Leave it to marinade for at least 2 hours or overnight, turning occasionally.
Once ready to cook, preheat the oven to 190°C/fan 170°C/gas mark 5. Place the chicken in a high sided roasting tray that the chicken fits snugly in and season the chicken well. Stuff the cavity with the quartered lemon, the whole garlic cloves and the remaining rosemary. Make sure there is a light layer of the marinade over it.
Place in the oven and roast for 1 hour 30 minutes, basting regularly, until golden brown all over and the juices run clear when inserted with a skewer. If the chicken starts browning too much, cover it with foil for the last 30 minutes. Remove from the oven and allow to rest for at least 15 minutes before carving.