Put the sugar, water and mirin in a small saucepan, bring to the boil and cook for 3 minutes until it starts to turn syrupy. Stir in the chilli, lime leaf and lime rind and add lime juice to taste. Set aside for 5 minutes.
Toss together the mango, radishes, cucumber and watercress and arrange on serving plates. Sprinkle the crab meat on top and then drizzle over the dressing.