Reduce heat to medium low and add the broad beans, courgettes, water and lemon slice. Stir well then bring to the boil over a high heat. Cover and reduce heat to medium low and leave to simmer gently for about 25 minutes, stirring occasionally. Cook until the courgettes are cooked and the insides of the beans are smooth and creamy. Add a few tbsps of water during cooking if it looks too dry.