Divide the prawns between 8 wooden skewers. In a shallow ceramic dish long enough to hold the skewers, beat together the grated rind and juice of the lemon and lime, the oil and garlic. Season with salt and pepper, add the prawn skewers and coat in the marinade. Cover and chill until required.
Very thinly slice the cucumber and put into a shallow dish. Sprinkle with the chilli, coriander, vinegar, fish sauce and sugar. Cover and chill until required.
When ready to serve, place the skewers under a preheated hot grill with the exposed parts of the skewers away from the heat. Cook for 7–8 minutes, turning once, until the prawns are bright pink all over. Stir the salad and spoon in to small bowls set on plates. Add two skewers to each plate, and garnish with coriander sprigs and lime wedges, if liked.