Cook lasagne in a large pot of boiling salted water until al dente. Drain well.
Preheat oven to 180 degrees C.
In a medium size saucepan over medium heat melt butter or margarine, stir in flour and let flour brown slightly. Stir in milk, stirring constantly until sauce thickens. Add the parmesan cheese to the sauce and stir well.
In a 20x30cm baking pan spread a thin layer of white sauce, followed by a layer of cooked lasagne. Place 1/4 of the prawns, 1/4 of the crab meat and 1 cup mozzarella cheese on top of the lasagne. Repeat three times, making four layers. Top the final layer with sauce and remaining mozzarella cheese. Bake for 45 minutes or until top is brown.