Line 4 prawn cocktail glasses with inner leaves of the iceberg lettuce. Mix the mayonnaise, yoghurt, milk, tomato ketchup, onions and most of the dill. Season to taste and add the brandy, lemon juice, and cayenne pepper. Stir in the prawns, tomatoes, peas and spoon the mixture into the lettuce-lined glasses. Sprinkle with a little cayenne pepper and dill and serve garnished with small sprigs of dill.