Whisk together the tomato ketchup, lime juice, soy sauce, radish, ginger, chile, sugar and coriander in a medium bowl and season lightly with salt and pepper. Cover and refrigerate for at least 30 minutes before serving. Can be made 8 hours in advance and refrigerated (do not add the coriander until ready to serve).
For the prawns:
Heat the griddle to high. Toss the prawns with the oil and season with salt and pepper. Place onto skewers and griddle for 1 to 2 minutes per side until lightly golden brown and just cooked through. Serve sauce in a bowl, surrounded by the prawns.