Combine the marinade ingredients in a nonmetallic dish. Add the prawns and stir well, then leave to marinate for 30 minutes. Meanwhile, soak 24 bamboo skewers in cold water for 30 minutes.
Thread one prawn on to each skewer and tip the marinade juices into a small pan.
Cook the prawns on a hot barbecue for 2 minutes on each side, then arrange on a platter. Heat the pan juices until they come to the boil, then pour over the prawns. Serve garnished with the herbs and chilli slices.