Place cornflour into a small bowl and add 1 tablespoon water to make a slurry. Add tomato sauce or Ketchup and whisk until well combined. Set aside.
Place caster sugar, white vinegar and water into a small saucepan and bring gently to the boil, stirring often or until sugar has dissolved.
Once sugar mixture comes to the boil, working quickly, pour in the cornflour/tomato mixture and whisk well until throughly combined and mixture starts to thicken. Remove immediately from heat and set aside at room temperature to cool completely. Once cool, use straight away or store in an airtight container in the fridge for up to 3 days.