Preheat the broiler and set the rack 6 to 8 inches from the heat. Place the English muffin halves, cut side up, on a baking sheet. Cook under the broiler until toasted, about 1 minute. Watch carefully so they don’t burn.
In a medium-sized bowl, stir together the salmon, yogurt, mayonnaise, bell pepper, celery, capers, pepper and salt.
Divide the salmon mixture between the 6 English muffin halves. Top with the tomato slices and Cheddar cheese slices.
Broil until the cheese is melted, about 2 minutes. Watch carefully so that the cheese doesn’t burn.