Fill a bowl with ice water. Stir sugar into a large pot filled with water; bring to the boil. Add corn; cook until kernels are softened, about 2 minutes. Drain and transfer corn to ice water until cooled; drain. Cut kernels from the cob with a sharp knife.
Combine corn, tomatoes, red pepper and spring onions in a bowl. Stir soured cream, mayonnaise, basil, Parmesan cheese and cider vinegar together in a different bowl until dressing is smooth; season with salt and pepper. Fold dressing into corn mixture. Chill in the fridge until flavours combine, 8 hours to overnight.