Remove the shells of the prawn leaving tails intact. Cut the prawns lengthways in half down to the tail, so that they flatten out to a symmetrical shape.
Thread a prawn on to 2 wooden skewers, with the tail between them. Thread another 3 prawns on to these 2 skewers in the same way. Lay the skewered prawns in a non-porous, non-metallic dish and sprinkle lime juice over it.
Mix the spices well with the oil. Coat the prawns well in the mixture. Cover it and chill for 4 hours. Cook over a hot barbecue or in a broiler pan lined with foil under a preheated broiler for 6 minutes, turning once. Serve hot with ketchup.