In a separate pan, melt butter or margarine. Remove pan from heat. Stir in flour and gradually blend in milk. Return pan to heat, stirring constantly. When liquid is smooth, bring sauce to the boil to thicken. Add half the cheese and parsley, with cayenne pepper. Season to taste. Skin and flake fish, and place in a bowl with prawns, eggs and sweetcorn (plus liquid from can). Stir in two-thirds of the sauce (reserve remainder), then spoon half the mixture over the base of a 4 pint / 2.25 litre shallow ovenproof dish. Cover with a layer of lasagne, repeat layers. Preheat oven to 180C/350F/Gas Mark 4. Spoon reserved sauce over lasagne. Mix together remaining cheese and parsley. Sprinkle over lasagne. Bake in preheated oven for 1 hour, until golden brown. Serve with salad.