Place the chicken breast side down on a chopping board. Using a good pair of kitchen scissors or poultry shears, cut along both sides of the backbone and down the sides of it, then cut along the bottom to remove this completely. Turn the chicken over and press hard to flatten it as much as possible (this will make it easier and faster to cook).
Mix all the spices with the oil until it makes a smooth paste, then rub this all over the chicken. Cover and leave this to marinade in the fridge for at least 1 hour.
Heat a barbecue or a griddle pan over a medium/low heat. Season the chicken and place it skin side down. Cook over a low heat for 15 -20 minutes until deep golden, then turn over and repeat on the other side. (You want this to be over a low heat but to be lightly sizzling – if it is too hot it will burn on the outside.) Add a splash of water, then cover with a barbecue lid or a roasting pan to completely cover it.
Continue to cook like this for 30-40 minutes depending on the size of the chicken, turning the chicken over every 10 minutes to make sure it doesn’t burn. The chicken will take 1 hour to 1 hour 15 minutes to cook depending on its size. Check the chicken by inserting a skewer into the middle – if clear juices run out, it is ready.
Transfer to a chopping board and leave to rest for around 10 minutes before serving. Sprinkle with the chillies to serve.