Stir cornflour and water in a small cup until cornflour dissolves; set aside. Bring milk and margarine to a boil in a saucepan stirring constantly. Reduce the heat and whisk the cornflour mixture into the milk mixture then bring to a simmer over medium heat. Cook and stir until the mixture is thick and smooth. Add the cheese to the milk mixture and stir until the cheese has melted and is well combined.
Prepare chilli as directed on the tin. Pour the cooked chilli and the cheese sauce over the top of the cooked chips.