Fill a pot with lightly salted water and bring to a rolling boil. Once the water is boiling, stir in the shell pasta and return to the boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, about 13 minutes. Drain well in a colander set in the sink and chill by rinsing with cold water.
Combine the cooled pasta, tuna, tomato, cucumber, red pepper, spring onions and celery in a salad bowl; toss lightly to mix. Stir in the ranch dressing and dill, cover and refrigerate until chilled, at least 1 hour.