Ask your butcher to spatchcock the chicken for you, or do it yourself if you know how.
Season the chicken with salt and pepper and rub with the oil. Thread two metal skewers through the bird diagonally (from wing tip to drumstick) to keep flat. Cook on a hot barbecue, breast-side down, for 5 minutes, then lower the heat to medium and cook, covered, for 15 minutes. Turn the chicken over and cook for a further 20 minutes. Set aside to rest for 10 minutes.
Meanwhile, spray the cut face of the lemon halves with a little oil and place them, cut-side down, on a hot barbecue. Cook for 10 minutes, turning frequently, until charred and softened. Set aside to cool.
Squeeze the lemon juice from two lemon halves into a bowl and stir in the garlic, oil, sugar, chilli, parsley and salt and pepper to taste.
Transfer the cooked bird to a board, cut into large pieces using a big knife and place on a warmed platter. Pour the dressing over it and serve garnished with some chilli and parsley and the remaining lemon halves.