Spatchcock the chicken! Use a sharp pair of kitchen scissors to cut out the backbone out of the chicken and flatten it out by pressing out so its flat.
Whisk together the ingredients for the marinade and coat the chicken in it. Leave for a few hours at least.
Meanwhile, whisk together the mayo and the sweet chilli.
When you are ready to eat your chicken, fire up your grill and fry the fries. Coat them in the chillis, sumac and salt and toss them thoroughly to coat - it helps when they are fresh out of the fryer!
Grill the chicken! Make sure your BBQ isn't too hot. It helps to move the coals over and use indirect heat and a BBQ lid to bake it. If it looks like your chicken will burn before being cooked through pop it in the oven to finish it off.