Put the strawberries, cherries, sugar and water on to boil on a moderate heat and keep stirring. You don't want this to stick to the bottom of the pan.
Take your choice of lemon or lime juice and mix it with the cornflour until it is smooth. Take the cherry mix off the boil and stir it in quickly until it smoothes out.
Put the mix back on the heat, stirring all the time while it bubbles to stop it burning. The idea is to bubble off the liquid until it reaches a thick consistency.
Leave the sauce to cool, then simply serve.