In a large Ziploc bag, combine the garlic, lemon juice, soy sauce, brown sugar, ketchup and sweet chilli sauce. Add the chicken thighs, seal and massage the bag to ensure each piece is coated completely. Refrigerate and marinate overnight.
Soak the bamboo skewers for at least one hour before using them.
Prepare the basting sauce by combining the soy sauce, brown sugar, ketchup and sweet chilli sauce.
Thread the chicken thigh onto the skewers, one piece of chicken on each stick.
Preheat grill or heat grill pan over medium heat. Grill the chicken, basting and turning every few minutes, until cooked through.