Put all of the ingredients for the butter into a food processor with some salt and whizz until smooth.
Use your hands to make a pocket for the butter by gently prying the skin away from the meat, starting from the neck end. Push the butter underneath the skin in a thick, even layer. Generously season the outside of the chicken and drizzle with a little oil. Leave covered at room temperature for 30 minutes.
Heat the oven to 200C/fan 180C/gas 6. Put the chicken into a roasting tin and cook for 1 hour 15 minutes, basting twice during cooking, or until the juices run clear when pierced at the thickest point. Move onto a plate and cover with foil. Rest for 20 minutes then serve with the resting juices and butter.