Dice the rhubarb and place in a saucepan with the sugar and 125 ml (4¼ fl oz/½ cup) of water. Bring to the boil, then remove from the heat and allow to cool. Transfer the mixture to an upright blender and blend until smooth. Pass through a fine sieve. Store in an airtight container for up to 3 days in the fridge. This recipe makes more than you will need.