Melt the butter in a large frying pan over medium heat. Cook onion, pepper, celery and garlic in butter until onions are tender. Stir in flour and cook for 5 to 10 minutes, stirring frequently. Stir in crayfish. Cover and simmer for 15 to 20 minutes, stirring often.
Stir in processed cheese, cream, Cajun seasonings and cayenne pepper. Cover and simmer for about 20 minutes, stirring occasionally.
Meanwhile, bring a large pot of lightly salted water to the boil. Cook pasta in boiling water for 8 to 10 minutes or until al dente; drain.
Preheat oven to 180 C / Gas 4. Butter a 20x30cm baking dish. Stir pasta into crayfish mixture; pour into prepared dish and sprinkle with Parmesan cheese.
Bake in a preheated oven for 20 minutes or until hot and bubbly.