Put 200g (generous 1 cup) of the blueberries (use the softer ones) into a pan with the sugar and 175ml (¾ cup) water. Slowly bring to the boil to dissolve the sugar, then simmer for 3-4 minutes. Mix the cornflour with the remaining water and add to the pan, stirring. Simmer, still stirring, for another 2-3 minutes. Remove from the heat and strain through a fine sieve int