Place chicken, chopped vegetables, bay leaf, salt and peppercorns in a large saucepan. Cover with water. Bring to a boil. Skim off surface with a slotted spoon. Cook, partly covered, over low heat 1 hour 15 minutes. Remove pan from heat. Leave chicken to cool in cooking liquid.
Separate chicken meat from carcass and chop flesh into small pieces; place in a bowl.
Trim pineapple, discard core and cut fruit into small pieces. Peel and segment 2 mandarins. Add pineapple and mandarin pieces to chicken. Cut leeks in half lengthwise and cut into very fine strips. Add to bowl.
To make curry dressing, whisk mayonnaise, yoghurt, orange juice and lemon juice until combined. Stir in curry powder, ginger and cayenne pepper. Add salt and pepper to taste. Stir dressing into chicken and pineapple mixture.
Peel and slice remaining mandarin crosswise. Tear or cut lettuce into wide strips. Distribute among serving bowls and top with chicken salad. Garnish with mandarin slices and lemon balm leaves.
Curry dressing is a versatile recipe. Use 4 tablespoons lime juice in place of orange juice and omit the lemon juice. Or, use a hot curry powder and omit the ginger.