Place the chicken fillets in a bowl with the curry paste and natural yogurt. Mix together to combine, then arrange on the rack of a foil-lined grill tray and slide under a preheated medium-hot grill. Grill for 7–8 minutes, turning once, until cooked through and lightly charred.
Meanwhile, spread the mango chutney or natural yogurt inside the ciabatta bread and top with the lettuce and sliced cucumber. Add the cooked chicken fillets, then cut the bread into 4 to serve.