Preheat the oven to 220°C/ Fan 200°C/Gas 7. Line a large baking tray with baking paper.
To spatchcock the chicken, place it breast-side down on a chopping board. Using a pair of poultry shears or heavy-duty kitchen scissors, cut down either side of the backbone and then remove the bone. Turn the chicken over breast-side up and gently press down on it with both hands until as flat as possible. Transfer the flattened chicken to the lined tray.
Mix all the marinade ingredients, except for the celery salt, in a small bowl. Rub the marinade all over the chicken and allow to marinate for 15 minutes.
Season with the celery salt, then roast for 40–45 minutes until deeply browned and cooked through. Remove from the oven, cover in foil and leave to rest for 10–15 minutes before serving.