In a small bowl, mix together the mayonnaise, double cream and ketchup until mixed. Add a few drops of Tabasco or a pinch of cayenne pepper.
Stir in the lemon juice and tarragon until well combined. Season to taste with freshly ground black pepper.
Stir in the prawns, reserving 4 to garnish.
Divide the shredded lettuce between glass dishes. Top with the prawn cocktail mixture and garnish each with a prawn and a pinch of cayenne pepper.