Place the tapioca in a bowl with enough water to cover. Leave it to soak for 8 hours, or overnight.
Fill a large saucepan with 2 litres water. Mix in the prunes, raisins, sugar and cinnamon. Bring to the boil, and stir in the soaked tapioca. Stirring often, cook 30 minutes, or until tapioca is clear and prunes are tender. Mix in the lemon juice and salt at the end of the cook time.
Chill until completely cooled before serving - it tastes better.