Heat your oven to 180ºC/Fan 160ºC/Gas 4.
Unwrap your chicken. Place it in a roasting tin and leave it to get up to room temperature.
Quarter your lemon. Squeeze half the lemon over the chicken. Toss the squeezed lemon pieces into the roasting tin. Squeeze the juice from the remaining two pieces into the cavity of the chicken, then chuck the squeezed lemons inside too.
Tuck a third of your thyme sprigs in the cavity. Slip another third of your thyme under the bird. Save the remaining sprigs to scatter over once the chicken is roasted.
Sprinkle the cajun spice and a good pinch of salt and pepper over the chicken and massage it all over the skin.
Cover the whole dish tightly with foil. Roast in the oven for 1 hr.
Remove the chicken from the oven. Crank the oven up to 220ºC/Fan 200ºC/Gas 7. Uncover the chicken. Baste with the dish juices.
Put the chicken back in the oven to cook for 20-30 mins or till golden. Check the chicken is cooked by piercing the thigh near the bone. The juices should run clear. If not, return to the oven and cook till they do.
Remove the chicken from the oven. Cover it with foil and set aside to rest for 30 mins to 1 hr before carving.
The pan juices are so good they make an instant gravy without needing to do anything else. So, once you carve, just drizzle the juices over the sliced meat, scatter over the remaining thyme leaves and get stuck in.