Heat a barbecue to medium.
1) Using a mortar and pestle, crush garlic cloves, peppercorns, coriander, lemongrass, and Thai chiles into a chucky paste. Using a large spoon, transfer the paste to a large mixing bowl. Add the chicken and the remaining ingredients and mix well. Cover and refrigerate for at least 2 hours or up to overnight.
2) Put the chicken on the barbecue, skin side down, and brush with the marinade. Barbecue for about 15 minutes then flip the chicken over and brush with the marinade. Barbecue the second side for about 15 minutes. Discard the marinade. When the chicken is cooked and tender transfer it to a serving platter and serve with warm dipping sauce.
For the dipping sauce:
1) Put all ingredients in a saucepan and put on the barbecue. Stir and bring to the boil. Move the pan to a cooler part of the barbecue and let the sauce simmer for 10 to 15 minutes, stirring occasionally. The sauce will reduce and become thick. Serve warm with chicken.