Preheat the oven to 190°C, fan 170°C, gas 5.
Mix together the ketchup, sugar, soy sauce and mustard and pour into a roasting tin. Add 4 pork sausages and toss in the mixture until fully coated. Roast for 30 minutes, turning the sausages once or twice during the cooking time.
Warm up 4 rolls in the oven for 3 minutes. Remove and cut them in half, leaving one side attached. Place one sausage in each roll, along with some salad leaves and a dollop of sweetcorn relish.