Boil the kettle and put the tea bags in a jug. Pour over 500ml boiling water and leave to steep for 5 mins. Meanwhile, put 100g blueberries in a jug, add the sugar and lightly crush with the end of a rolling pin or a potato masher. Remove the tea bags from the water, pour the tea over the blueberries and top up with another 300ml cold water. Add a large handful of ice to cool quickly, or chill until cold.
When you’re ready to pack your picnic, pour the iced tea into bottles or flasks. Add a few sprigs of fresh mint, some lemon slices and the remaining blueberries. Seal and store in a cooler bag.