In a large pot of salted boiling water, cook pasta until al dente, rinse under cold water and drain.
In a medium bowl combine mayonnaise, soured cream, milk and Italian dressing mix. Whisk together until smooth, set aside.
In a large salad bowl combine pasta, peas, olives, salami, spring onions, celery and parsley. Mix in dressing last, reserving 125ml. Let sit overnight in the refrigerator. Stir before serving. Add extra dressing if pasta appears dry.