Make the lemon rind and juice up to 250
ml/8 fl oz/1 cup with water. Place in a saucepan with the sugar. Heat
gently, stirring, until the sugar dissolves, then bring to the boil
and boil for 3 minutes. Stir in the banana. Puree in a blender or food
processor and return to the pan. Blend the arrowroot with the amaretto
and stir into the pan. Bring to the boil, stirring, until thickened.