Bring a large pot of lightly salted water to a boil. Add pasta and cook until tender, about 8 minutes. Drain and rinse under cold running water to cool.
In a serving bowl combine the mayonnaise, sour cream, lemon juice, mustard, dill and black pepper. Gently stir in the pasta, prawns, celery, cucumber, tomato and eschallot. Mix in salt to taste and refrigerate for at least 2 hours before serving.