Put the cherries in a saucepan with the caster sugar and water.
Bring to the boil and simmer for 5 minutes. (If using canned cherries replace the water and sugar with the syrup from the can.)
Dilute the arrowroot with a little water, add to the cherries and simmer for a further 3-4 minutes, stirring occasionally.
Transfer into a bowl, lay a piece of clingfilm on the surf