Preheat a char grill or barbecue over high heat. Remove prawn heads & veins, brush prawns with oil & cook for 2-3 minutes each side. Set aside & keep warm.
Place the fish sauce, caster sugar, water & lime leaves into a small frying pan over high heat, bring to the boil, add peanuts & simmer for 1-2 minutes or until thickened. Pour over prawns, add coriander & toss to combine.
Serve with lots of fresh lime wedges & extra coriander.