Place cherries, sugar and water into a medium saucepan and bring to a simmer.
Simmer for 2 minutes (if using fresh cherries you may need a few more minutes to slightly soften the cherries)
Mix cornstarch (cornflour) with water in a small bowl.
Lower the heat on the cherries, then pour in the cornstarch and water stirring all the time.
Increase heat to a simmer.
Cook until thick and clear about 2 minutes.
Pour into a small bowl to cool or glass jar to store in the refrigerator
Using canned cherries.
Yes, it's even more simple if you use canned cherries. The juice in the canned cherries is substituted for the water, lemon juice, and sugar. To make Cherry sauce with canned cherries here's what you do.
Use two 15ounce cans/415gram of pitted black cherries in syrup.
Heat in a saucepan until simmering.
Add cornstarch/cornflour slurry as per the original recipe.